6. It leaves us with no grounds for opposing any form of consensual intercourse among adults.  No culture in history has accepted (even celebrated!) homosexual acts between adult men or adult women—not even the ancient Greeks. But plenty of cultures have accepted polygamy, the marriage of one man to several wives. Certain religions allow it or encourage it: Islam allows a man to have up to four wives, and radical Mormonism is, as I understand it, even more generous.

There are natural justifications for it. A rich man can thereby father, and support, dozens of children; the tribe benefits from the fecundity. A man can beget several children virtually at once. An older and well-established man can continue to father children long after his first wife has grown too old to bear them. As I say, it is culturally common; not as common as monogamy, but common enough not to surprise.

What grounds could we have to deny people the opportunity to marry more than one person? If we establish as a matter of law that marital relations are free to any two people who consent, why limit the number to two? Polygamy, after all, is much easier to justify than are homosexual relations: It does not violate the biology of the people involved; it brings forth many children; it preserves the ideal of the union of male and female.

But what would happen if the door were opened to polygamy? Would we not find ourselves in a world utterly different from the one into which we were born? One might say, “I do not believe in it; I will never marry another.” But what about one’s spouse? What about the members of the opposite sex whom you may happen to meet? In every culture that allows polygamy, the pressure of the possibility of dalliance and marriage, no matter who you are (for it turns married men instantly into eligible bachelors), compels a severe separation of roles for men and women. Is that what we want?

How could we deny any combination of people who wish to “marry”? What of two so-called bisexual men, who want to “marry” one another and their shared wife? If homosexuals claim rights based upon their sexual actions, why not bisexuals? If the marital act is all about the fulfillment of one’s desires, how can even homosexual pseudogamy fulfill the bisexual? Does he or she not have the same “right” to fulfillment as the homosexual? On what grounds could we deny a marriage license to an adult brother and sister? Reasons of health? Not if one or both have been sterilized.

Why stop here? What about people whose desires cannot be fulfilled unless they perform sexual actions in public? Or with animals? Or with precocious children? Logic requires an answer. If you affirm the false principle, you must go where that principle leads. As for now, the only thing preventing the collapse of all sexual constraints is a residual feeling of disgust. That is one rickety door to batter down.