Oh, I get it. You’re not here to be enlightened, you just wanna argue. Fine, I’ll bite.
For everything that exists in nature, there exists a more perfect “form” or “essence” or “nature” in which it is said to “participate” or “instantiate.” For example, the essence of a triangle is to be a closed-plane shape consisting of three straight lines. Anything with this essence has a number of properties that follow logically from it, such as having three angles that sum to 180 degrees. It is possible for a thing to participate imperfectly in an essence; for instance, if I hastily scrawl a triangle on a sheet of paper on a bumpy surface with a worn-down crayon, it is likely to have non-straight lines and thus angles that sum to something other than 180 degrees. We would not say that such a shape is not a triangle, even though it clearly does not live up to that essence. We would simply say it’s a “bad” triangle.
The badness of the triangle is not an arbitrary judgment; it is an objective fact, arising from the nature of what a triangle is and what it means to be triangular. (The nature of the triangle is an objective fact, too; we didn’t make up the definition I provided above, we discovered it).
Man, too, has an essence. For instance, it’s appropriate for me to say “human beings have two legs.” Obviously I’m not talking about each and every human being alive. Rather, I’m talking about human beings in principle. We recognize that being born with only one leg is a birth defect for precisely that reason; it cannot be a defect unless it is a failure to instantiate the essence of something.
The same principle, applied to human behavior, furnishes a basis for morality. We have various faculties; these faculties point to an end; goodness therefore consists in using our faculties in a manner consistent with their respective ends, and evil in using them for some contrary purpose. For instance, we have reason; reason points toward the end of discerning truth; therefore it is good to discern truth and bad for us to, say, invent and disseminate falsehoods.
Our sexual faculty points to the end of procreation. We know this because the sexual act concludes with the emission of semen, and the emission of semen causes pregnancy. Humans have the distinct genital configuration they do because it maximizes the possibility of conception, and we have a strong sex drive so that we will have sex often and therefore maximize the possibility of conception, and it is very easy to get pregnant during a woman’s fertile period so as to maximize the possibility of conception, and so on. If our sexual faculty points to the end of procreation, and if goodness consists in using our faculties in a manner consistent with their respective ends, then it is good to have sex in a manner that is conducive to procreation and not good (evil, sinful, etc.) to have sex in a manner contrary to the end of procreation. Hence the Church’s prohibition on contraception, sodomy, masturbation, etc.
The sexual act points toward the end of procreation, but obviously procreation does not end at conception. The pregnant woman is vulnerable and in need of care, and her child, once born, will also be in need of care as well as instruction and proper upbringing. The sexual act therefore necessitates a measure of commitment and obligation. It is from this fact that the institution of marriage from a natural standpoint logically arises.
Civil marriage sprang up around the natural institution of marriage, for the same reason but for slightly different justifications. The state’s interest is not in promoting morality but in promoting the common good, specifically, the care and disposition of children. Thus civil marriage provides a legal framework within which couples can discharge their duty to one another and to the community by creating new life and ensuring that that new life is cared for.
Homosexual unions are intrinsically incapable of producing children and so are never a matter of public interest – hardly worthy of the endorsement of the state.
QED.
