Commenting on our recent Corner exchange on gay marriage, Andrew Sullivan inadvertently makes a critical point against gay marriage. Imagine there were no social costs for adultery. In that case, says Sullivan, heterosexual promiscuity would soar. But this is precisely the danger of gay marriage. Sullivan argues that gay marriage will reduce homosexual promiscuity. But what if the effect goes in reverse? What if the many gays who believe in sexually open relationships undermine the marital ethos of monogamy? In that case, the dangers to heterosexual marriage that Sullivan himself outlines would result. So a great deal turns on what the real effect of gay marriage on monogamy will be. The problem is, it’s really women who reduce male promiscuity, not marriage itself. Marriage can reinforce the domesticating effect of heterosexual coupling, but it cannot create that effect out of whole cloth. So gay marriage will not have the results that Sullivan thinks. But obviously, given Sullivan’s own logic, gay marriage’s effect on monogamy is the crucial question, and it needs to be debated. Yet the mainstream media won’t even raise the issue.