Nathan Smith conveys the classical understanding of sex:

Men are tempted to exploit women for pleasure and prestige, and need to be on their guard against this temptation. Exploitation is worst when the woman is underage or drunk or emotionally unstable, or when the man uses a position of power to intimidate her, tells lies to impress her, promises to marry her, conceals his marriage to someone else, gets her pregnant, or exposes her to a sexually transmitted disease. But the bottom line is that if he serves his own pleasure at the expense of her welfare, that’s exploitation. If he knew, or could have known if he thought about it, that she’d regret it the morning after, that’s exploitation. And if he knew, or could have known, that she’d regret it one year, or five years, or fifteen years later, when she’s wasted some or most or all of her remaining reproductive years on a guy who wouldn’t marry her, that’s exploitation, too. “He used me” is a standard—and just, and accurate—complaint made by women against men they’ve had sex with. … The only ethically safe course is either to marry a woman or else to leave her chastity intact. …

Humans are intensely ambivalent about sex, regarding it by turns as vulgar, gross, and unseemly, or as sublime and beautiful. We place rape among the worst of crimes, while romantic love is one of life’s crowning glories, the theme of half the novels and songs the human race has written. The deceit and damage involved in so much premarital sex—cool dude bangs insecure girl and turns her into a single mom on welfare for life—fully justifies the repugnance that is one side of this ambivalence.

On the other side is the glory of marriage, and while there’s more to that glory than the selfish genes can explain, they shed an important light on it. For when two people marry, “leaving father and mother” as the Bible says and committing to lifelong monogamy, their genetic interests are united, at least approximately, creating a harmony of instincts. Ordinarily, our instincts put us in competition with our fellow human beings. In marriage, instinct is on the side of love.

Children are the large, obvious reason why marriage is good for society and why premarital sex isn’t. Sexual relationships always absorb a lot of people’s energy and attention, so they impoverish society unless they give something back. Marriage makes the next generation, under the most favorable conditions. Premarital sex is usually not intended for procreation, and if it does result in children, they enter life at a disadvantage because they lack stable parental commitments to raising them.

But even compared to childless marriage, premarital sex has an unwholesome character because, by failing to address genetic conflicts of interest through marriage, it allows competition, exploitation, and fear of betrayal to penetrate into the heart of the most intimate human relationships, not stealthily, but openly and as if by right. There is no way to make premarital sex promote the good of society or of the individuals involved. The world would be a better place if it never happened at all.

If this perspective seems outrageous or even just incredibly distant to you, that’s an example of how revolutionary the Sexual Revolution of the 1960s really turned out to be. The ethics of human sexuality can be far richer than our present “consent” culture allows for.