The untutored or perhaps commonsense view is that there's something out of order about certain sexual acts. They're "unnatural," as people once said, or "intrinsically disordered," as the Church says today.
Nowadays of course educated people think that's all ridiculous. After all, no one says sodomy or whatnot is miraculous, so it evidently complies with the order of nature. And all intentional actions are unnatural in some sense, since they change what would happen if we let things go their own way.
So to discuss sexual conduct with people today it seems you have to go with the flow and start with the assumption that there's no natural or unnatural in human affairs. Still, there's less to the change than meets the eye. "Free to be you and me" is not the sum of all wisdom. There's still the question how choice should be exercised.
Our dealings with everyone and everything can certainly be understood as a matter of choice. But it's a good idea (for example) to have some definite system of property and government, and some systems work much better than others. The world may be what we make it, but some choices work out well and others badly, and we should choose the good and leave aside the bad.
So it's not clear what's gained by denying that some things are more natural than others. Naturam expellas furca, tamen usque recurret--you may drive nature out with a pitchfork, but she will keep coming back. Whatever people say, they still have to deal with life in ways that make sense. That means they have to know what things are and how they work--that is, they have to understand their nature and notice what goes with the grain and what goes against it.
The question as to sex seems to be how it can be civilized and made part of the pattern of a good life. It's evident something has to be done. Like anger or the desire for power or gain, it's a strong impulse that's not self-regulating. It can wander off in all sorts of directions that cause big problems. If it becomes obsessive we're not happy and if we make it trivial we're not happy either.
So far as I can tell, the universal solution has been to tie it into a functional pattern that tells us what it is and what it's for so we can know what to do about it and it can add to human life and order it rather than disrupt it. The solution hasn't been applied with strict consistency, since sex is hard to nail down and people do odd things, but it's always been fundamental. Otherwise marriage would not be a universal institution.
What kind of pattern is needed though? The solution won't work if it's an arbitrary one that people can make into anything they want. To say that it's the nature of sex to serve the individual's scheme of fulfillment, for example, would leave it with no definite point or function. It would remain whatever someone makes of it at the moment.
Sex is too basic and affects the interests of other people and our own psychology too deeply to treat that way. It constructs the world we inhabit, so it can't be simply a routine object of choice within that world. For that reason it seems that the general pattern for dealing with things and people in modern America--property, contract, and consumer sovereignty--can't really apply.
That's not surprising, since property and contract never apply to the most basic things. For example, they don't apply to themselves. No one owns the rules of property and we don't contract into them (John Locke to the contrary notwithstanding). The same is true of contract.
Ditto for all basic human connections--friendship, marriage, nationality. We can't simply decide what those things will be. As Hegel points out, marriage starts in contract but it's a contract to transcend the standpoint of contract. To treat it as simply a matter of contract or property violates it. Ditto for friendship and nationality. They're not objects of trade or ownership and we can't turn them at will into this or that.
Sex is no less basic than property, contract, friendship, marriage, and nationality. It's how we come to be and it determines who we are and our most important relations to others--man, woman, mother, father, husband, wife, brother, sister, son, daughter. Those relations aren't bureaucratic or commercial, so current ways of thinking can't deal with them, but they're nonetheless basic to what we are and our position in the world.
Sex is therefore a serious part of how things work. As such, it has a natural function with innumerable tendencies and connections that seem innate and determine what it is and what to do about it. If you accept its essential connection to procreation, for example, then it's clear that its very nature ties it to loyalty, acceptance of responsibility, love that goes beyond momentary feeling, and so on. The connection means that it points beyond itself, so the absence of loyalty is a violation of what it is.
If you tie sex to procreation you can understand marriage and family life. Otherwise you can't. But what does it mean to tie sex to procreation? People today interpret the asserted connection technologically, because they think technology exhausts reason. They think that if it means anything it means sex has to be treated as a reproductive technique.
That's absurd and a bit creepy, so people reject the idea of functional connection. They believe the essence of humanity is self-definition, so sex is humanized by making it a matter of arbitrary will. We're back to free to be you and me.
They would do better to reject the idea that technique and willfulness are the whole of life. Sex is a human relationship, and we should view human relations humanly. That means we should take their function seriously but not view it technologically.
Consider friendship, marriage, and nationality. If those relationships had no function they'd have no particular content and there'd be no way to take them seriously. They wouldn't be what they are. So they do have functions, but that doesn't make them means to something else.
It's not human to turn human relations into pure means. That's why those relations don't normally disappear if some particular thing happens to frustrate their general function. Friends help friends, husbands and wives support each other, and citizens support their country, but you don't stop being a friend, husband, wife, or citizen if you're lying deathly ill in prison and can't do much of anything for anybody. You stop when you give up the connection or intentionally do something at odds with its nature.
The same applies to sex. Masculinity and femininity are basic to sex and to who we are. The natural function of sexual union is procreation. So that union is part of a functional system that engages the identity of the parties and involves the creation of new life. That remains true even if some particular thing--time of month, age or physical condition of the parties--happens to prevent new life from coming about. What matters is the identity of the act and parties.
People who think about things technologically--educated Western people who go along with what they've been told all their lives--can't make sense of that view of things. Ordinary human ways of thinking that involve concepts like identity don't have the rigor they've been taught to view as essential to rationality, so they see them as subjective expressions of inherited prejudice. "Gay marriage" has the same practical effect as the marriage of two 60-year-olds, so they think it's crazy to treat it differently.
So much the worse for educated Westerners. The technological way of thinking about human affairs isn't human, isn't adequate, and isn't going to last. We are not rational economic calculators, and that's not a bug but a feature. We understand ourselves, others, and our actions by reference to what we and they are. The question for living is what understanding of identity and function best serves the best purposes of human life.
What is man? What is woman? What is life? Those are big questions, but they are unavoidable, and pure cogitation can't deal with them. In the long run we can't live by modern rationalism. People think they're stuck with it, because they think the alternative is arbitrariness and irrationality. It would be better if they opened their eyes and showed more respect for how people have always lived--that is, for human nature.
A better way forward is renewed acceptance of tradition as the means by which fragmentary arguments and partial understandings come together to become part of an overall way of life with definite institutions, orientation, and standards. That is how men have always lived, and it is part of what it means to be social and cultural. Only by such means can we live a truly human life.