David Corn reports on the exchange:

Santorum has been in this spot before, and he easily adopted a here-we-go-again stance, and, in a somewhat condescending manner, struck back with…logic. Or what he claimed to be logic. He asked the students to justify gay marriage. When one said, "How about the idea that all men are created equal and [have] the right to happiness and liberty," Santorum asked, Are you saying that everyone should have the right to marry anyone?

The student said yes. And Santorum quickly retorted. "So anyone can marry several people?"

No, the student said.

But what if someone can only be happy if he or she was married to five people? Santorum asked her.

Others in the crowd starting jeering him. "That's not the point," one shouted.

But Santorum, who kept cutting off the students, stuck to this argument. When the students talked about equal rights, he repeatedly interrupted, "What about three men?"

That Santorum has regurgitated the polygamy point reveals, it seems to me, the weakness of his thinking on this issue. Gay people are not seeking the right to marry anyone. They are seeking the right merely to marry someone. Currently we are denied that basic civil right that every heterosexual takes completely for granted, in most states. The issue of polygamy is completely different and separate. Currently, no straight people have a right to a three-way marriage, let alone gay ones. I think there are very good social reasons for that, although it's certainly worth debating. But it's another debate. The only way Santorum's argument works is with the premise that gays denied any right to marry are denied no right at all. They are not in the same category as heterosexuals, and their relationships, and the benefits they bring, are inherently inferior, indeed morally repugnant.

That's Santorum's view. It's his view that private gay sex can and should be regulated by the government to prevent the evil of sodomy from destroying society. And sodomy, remember, means any non-procreative sex act: oral sex, masturbation and, worst of all, contraception: a deliberate flouting of natural law. If this is the position of the GOP, it is essentially turning itself into an irrelevance for the vast majority of those under 40, and hefty proportion of everyone above.

The boos are a harbinger. But they may turn to cheers, of course, in South Carolina.