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.