I was asked a question by a young college student who happens to be Catholic regarding the Church teaching on homosexuality and how it coheres with the rest of Church teaching. His impression was that the teaching just caused a lot of needless disunity and suffering.
Obviously, the Catholic Church teaches a lot of things which are seen to cause disunity and suffering in the current age, and we would do well to include the whole of Catholic teaching on human sexuality in that category. After all, what does the teaching on premarital sex or contraception do if not cause disunity and suffering?
Aside from questioning the assumption that beliefs are the source of disunity, which seems odd given that the Catholic Church sees belief as a major source of Church unity (and indeed defines being in communion with the Catholic Church), I thought it best to address the topic holistically, showing where the teaching on homosexuality coheres with Church teaching and why changing that teaching would in fact be deeply incoherent.
You may note that I have not mentioned the Theology of the Body because it is a relatively recent development.
There are three aspects of the Church with which its teaching on homosexual behavior coheres very well: asceticism, Natural Law, and Sacred Scripture. Taken by themselves and viewed within the confines of contemporary philosophical assumptions, they are not convincing. But for someone who accepts a lot of traditional philosophical assumptions and operates within a Catholic context, they will probably be more compelling as a set of coherent influences.
The first aspect I will address is asceticism, and as a disclaimer this is the one I find most compelling on a personal level because I'm an ascetic.
Asceticism has been a component of Christianity from the beginning. For example, John the Baptist very much lived the life of an ascetic. When Jesus sent the Apostles out to preach the Gospel, they were given the guidelines for a very simple and ascetic sort of life. This also coheres very well with existing ascetic traditions in Judaism existing contemporaneously with Jesus as well as older ascetic traditions within Judaism. For more information about asceticism in Christianity and other forms of asceticism, I've provided a link above.
Ascetics all over the world and throughout human history, religious and irreligious alike, have realized that to practice virtue consistently a high degree of control over the self is a prerequisite. And the way we gain the self-control necessary is to practice self-denial. We habituate our bodies to being in control of our will by this discipline of self-management, which includes practicing penances for our failures and denying our less than virtuous desires from being actualized.
To gain the most potent and important form of self-control, we must deny our most basic desires, which include our desires for food and for sexual gratification. The ascetic does not view food or sex as being intrinsically evil; they have legitimate purposes as sustenance or procreative activity. It is rather that our self-indulgence in those things is evil.
So how does this apply to the case of same-sex unions? Both persons of heterosexual and homosexual inclination are called to live chastely and to give up self-indulgence in favor of purposes which are unselfish and sacrificial. In consequence, this means that both are called to reject recreational sexual activity in favor of procreational sexual activity. Married couples in the Catholic Church are obliged to abstain from masturbation, oral sex, anal sex, and any other sexual activity which would be for purely self-indulgent purposes (1). The one type of sexual activity which is encouraged is the type that is open to procreation, and for biological reasons this is obviously not a type of sexual activity one can engage in with a member of the same sex.
While asceticism is certainly a strong influence on Catholic moral teaching, it is not the only influence. Some ascetic practices are a basic part of Catholic moral teaching and all Catholics are indeed obligated to them, though it's readily apparent in the industrialized Western world that a lot of Catholics don't follow those teachings very well. (When overindulgence is the baseline, it's hard to get people to realize what healthy living means.)
We see this required asceticism in the universal practice of fasting and the moral teaching on sexuality. Other moral teachings, such as the teachings regarding murder or theft, have to do with other moral imperatives and really are not influenced by asceticism. But they are explained by...
Now let's attend to the topic of Natural Law and how it coheres with the aforementioned principles of asceticism and Catholic moral teaching. You'll notice that asceticism and Natural Law are essentially in agreement on the question of which moral acts are good though they come at the issue from different philosophical considerations.
As a disclaimer, I have some philosophical issues with Natural Law theory. But that does not keep me from recognizing that it is an elegant and powerful moral theory which is worth examining for its usefulness and uniqueness.
As you undoubtedly know, Natural Law theory concludes that homosexual intercourse is not in alignment with the proper end of a human being (2). But how does it get there?
For Aquinas, we would need to evaluate the moral quality of an act via its object, ends, and circumstances (SEP, CCC 1750-1754). As we can see, the Catechism essentially adopts the Natural Law perspective of Aquinas as a means of clarifying the Christian moral philosophy.
Conveniently, many of the common contemporary objections to Church teaching on homosexuality (within Christianity) tend to fall into the categories of objections based on object, ends, and circumstances.
The key question with regard to the object of homosexual acts is whether or not homosexual acts are a true good. The objection here is usually that there are good things about homosexual acts, and those good things would be the pleasure and the feelings of unity and/or love. Aquinas recognized that many human behaviors were apparent goods, that we apprehend things as good which are not fully in alignment with our proper end. This would be the case with many behaviors such as masturbation, pre-marital sex, extra-marital sex, recreational drug use, etc. To give an example of why the argument from apparent goods doesn't work in a Catholic context, let's consider the case of pre-marital sex. The same set of apparent goods present in the case of homosexual acts exist in the case of pre-marital sex as well. And pre-marital sex has an even greater advantage from a Natural Law standpoint: it is in alignment with our procreative purpose. It is nonetheless proscribed. If we accept the argument that the aforementioned apparent goods justify homosexual acts, then we justify pre-marital sex between committed heterosexual partners as well. Far from being coherent with Church teaching, this would be wildly incoherent with Church teaching.
The objection raised regarding the ends of homosexual acts is that the intentions of those seeking to engage in homosexual acts are often positive. And from a Natural Law perspective, this would go back to the issue of apparent goods. It's certainly fair to say that many people seeking to engage in homosexual acts are seeking an apparent good, which is an entirely understandable thing to do. Nonetheless, the argument from good intentions fails to be compelling for the obvious reason that even the most clearly morally depraved actions might be and sometimes are undertaken with the best of intentions. In Catholic moral philosophy, the end does not justify the means (CCC 1750 - 1754). Both end and means need to be in alignment with our telos.
The objection raised regarding circumstances is often that homosexual acts are morally acceptable under the circumstances of a committed relationship. Not only is this obviously incoherent with the general prohibition on pre-marital sex, but circumstances in Natural Law are not what determines whether an act is morally acceptable; circumstances merely alter the degree of culpability or responsibility of a moral agent for an act (CCC 1754).
There are more general objections to Natural Law, but my primary purpose was to show how well Natural Law coheres with Church teaching and how the common objections decidedly do not.
I'd like to begin the third topic with another disclaimer. I notice that most Christians who make an argument from Sacred Scripture regarding whether or not homosexual acts are prohibited engage in a heavy dose of confirmation bias. In the case of those who are arguing for prohibiting homosexual acts in a Christian context, they tend to overestimate the weight of the evidence. In the case of those who are arguing for treating homosexual acts as morally acceptable in a Christian context, they tend to underestimate the weight of the evidence.
Most arguments for prohibition begin with the Old Testament law against a man lying with a man as with a woman. Coming from most Christians of Gentile origin, this is an argument that's hard to give full and crushing weight. The previous covenant is not what binds the Christians of Gentile origin. That said, Christians are not in a position to dismiss the moral principles in the Torah without due consideration. We certainly give weight to the Ten Commandments and other moral obligations therein, evaluating them in light of Christian theology and traditional practice. And to the extent that Christian theology and traditional practice supports the proscriptions and prescriptions of the Old Testament, we typically hold them as binding.
Another commonly cited verse is in the epistles of the New Testament, specifically Romans. It's not a clear prohibition of homosexual acts, and we shouldn't read it that way. However, it is fairly clear when we look at the context that the author is listing those acts among lots of other sinful acts. At the very least, the author's basic assumption is that homosexual acts are sinful, and the author's intent is certainly relevant in the Catholic Church's approach to scripture.
None of this adds up to a crystal clear prohibition on homosexual acts, but it does lead us to not being able to take seriously the idea that homosexuality is acceptable in a Christian context (3), and it coheres very well with the aforementioned tradition of asceticism as well as Natural Law.
(1) It does not follow that enjoyment should not be a factor in sex. From the perspective of an ascetic, there's nothing wrong with enjoying sex so long as the enjoyment of the act (for you or for your partner) is not the primary function of having sex. It could certainly be a latent function of sex.
That said, we can safely say that there's no particular need for it to be enjoyable every time. And many married couples can and will attest to the fact that it's not enjoyable every time. For example, a husband not infrequently comes home completely exhausted and just wants to eat and go to bed. But his wife wants to have another child, and his love for her allows him to set aside his own fatigue so that they can try to have another child. Would you say that he's violated any moral imperatives, or that he should not have sex with her until he's able to properly enjoy it?
(2) To respond to the objection that homosexuality is natural, let's talk about necessary and sufficient conditions for a moment. Natural Law proposes a procreative telos as a general norm. This has two effects, and the first is to rule out sexual acts which have no procreative potential as moral goods. The second effect is to establish a necessary condition for sexual acts to be morally acceptable, but it is not sufficient for it. If it were a sufficient condition, pre-marital sex would be justified in the eyes of Natural Law, and it is most decidedly not.
So is not pre-marital sex also very natural? Do we not see it commonly in other species? Mating for life is not common in other animals by a long shot.
Natural Law does not make the claim that natural = good, though a lot of folks make the mistake of interpreting it to mean that. It also rejects the claim that what is good for a human is what is good for other beings. Natural Law sees various beings as having a telos which is specific to that being, and in particular sees humans as having a very different telos because we are a different kind of animal. We might fairly say that we have different moral obligations because we have different faculties.
Not only would it not make any sense to make the argument that what is natural is morally acceptable in a Natural Law framework, but if we were to propose it as a standard, then every action becomes acceptable because all of our vices are natural. It would destroy any moral framework completely by collapsing the is-ought distinction.
(3) Perhaps I need to clarify different ways in which evidence can function with regard to a particular claim.
Evidence can be used to positively verify a claim, but this requires an extremely high degree of certainty. Evidence can also be used to falsify a claim, and this is a much easier burden to meet. To falsify a claim does not prove that the opposing claim is verified. For example, falsifying the claim that the Earth is flat does not verify the claim that the Earth is a perfect sphere.
In the case of Sacred Scripture, I don't think we have quite the weight of evidence required to verify the claim that "homosexual acts are clearly prohibited", but I do think that we have enough evidence to falsify the claim that "homosexual acts are acceptable" in a Christian context.
This rules out proclaiming homosexual acts as acceptable, but we have to appeal to the aspects of Christian tradition discussed previously to get to the point where we can state definitively that homosexual acts are prohibited in a Christian context.