CHICAGO - Life is full of zero-sum games, where one person can't win unless another loses. If I get the last piece of pie, you don't. This is what makes governing so much a matter of choosing among painful tradeoffs.
If a new highway is built over here, for the good or ill of Group A, it can't be built over there, for the good or ill of Group B. So the decision may pit the two groups in a fierce fight for their mutually exclusive interests.
The battle over gay marriage is as bitter as you would expect in a zero-sum game between two groups with incompatible desires. Conservatives want to preserve traditional marriage as the foundation of our society. Homosexuals want the legal recognition, rights and duties that go with matrimony. They can't both get their way, can they?
Actually, they can - which is why the rabid opposition has always been a bit puzzling. This is a case in which one group can be accommodated without any sacrifice from the other. Letting gays marry doesn't damage what heterosexuals have, any more than giving blacks the vote meant taking it away from whites.
But conservative commentators, alarmed by Canada's decision to allow gay marriage, see the change as an attack on a vital institution. David Frum, a former speechwriter for President Bush, worries that gay marriage "abolishes marriage and puts a new, flimsier institution in its place."
Hudson Institute scholar Stanley Kurtz says, "The need to protect marriage obligates us to reject so radical and dangerous a social experiment." Patrick J. Buchanan warns it would promote "a death-style that appears to cut decades off the life of those who indulge in it."
The critics are badly confused. In truth, the flimsier version of marriage is already in place, the radical social experiment has become the norm, and unprotected gay sex managed to spread the AIDS virus without any help from gay marriage, thank you.
The crucial change was the sexual revolution - which freed people to follow their own moral code on these matters without fear of punishment. The widespread availability of sex for both men and women removed one major motive for marrying and one major disadvantage of divorce. Both changes undoubtedly weakened the institution.
In 1972, almost half of Americans said they disapproved of premarital sex "always" or "almost always." Today, 64 percent generally approve of it. Regardless of what people say, almost everyone practices it. Virginal brides today are now as rare as communist dictators. Couples are also far more likely to live together without getting married.
Once heterosexuals had slipped the bonds of strict sexual morality, homosexuals were bound to follow.
So when conservatives act as though gay marriage would be a radical change, they're mistaken. The radical change is already a fact of life. Gays, like straights, are now free to frolic with whomever they choose, set up housekeeping with a partner and even raise children together. They don't have to get married to do any of these. Cohabitation is a flimsier version of marriage, and it didn't happen because a legislature made it available.
What created the push for gay marriage is that many homosexuals, like most heterosexuals, aren't willing to settle for the flimsier version, or for mere sexual liberty. They want all the obligations and prerogatives available to husbands and wives.
How can that be bad? If the women on Sex and the City marched to the altar, traditionalists would rejoice at their decision to abandon casual affairs in favor of permanent commitments. But when gays want to embrace old-fashioned monogamy, traditionalists are horrified.
They shouldn't be. Gay marriage would encourage gays to accept a more - what's the word? - conservative lifestyle. It would also strengthen families that already exist. Many established gay couples have the task of raising children, who would be better off with parents who are legally joined. Conservatives may think gay households are not the best place for children. But the fact is, the kids are already there.
Denying marriage to gays won't prevent homosexuality, and it won't ameliorate the ills that critics associate with it. The attitude of opponents is like noticing that sick people often take antibiotics - and deciding that if people were denied antibiotics, they wouldn't get sick. Gay marriage isn't a repudiation of the values conservative prize. It's an affirmation.
Steve Chapman is a columnist for the Chicago Tribune, a Tribune Publishing company. His column appears Tuesdays and Fridays in The Sun.