Ah, the argument by analogy — its potential for backfire knows few limits.  After attorneys for Pennsylvania Governor Tom Corbett, a first-term Republican vying for re-election, suggested that same-sex marriage would be the equivalent of allowing 12-year-olds to marry, Corbett tried to distance himself from the remarks.  “I think a much better analogy,” Corbett told WHP-TV in Harrisburg, “would have been brother and sister, don’t you?”

“I’m going to leave the comments to you and your team,” the interviewer says with a laugh, refusing to get drawn into the what she clearly knew would be an eruption of outrage. And of course it followed soon after:

Mark Aronchick, a lawyer for the plaintiffs in that case, called Corbett’s remarks “insensitive, insulting and plainly wrong.”

“In other words, some kind of incestuous relationship,” Aronchick said. “He’s just out of touch on this one. Gay people marry for the same reasons straight people do — to express their love and to declare their commitment before friends and family.”

A Corbett spokesman offered no immediate comment Friday morning.

Ted Martin with Equality Pennsylvania, which advocates on behalf of lesbian, gay, bisexual and transgender people, called the governor’s remarks “shocking and hurtful” and asked him to apologize.

The replacement analogy was unfortunate politically, and it’s not even defensible philosophically. The issue of incest has nothing to do with the model of marriage. Why not stick to arguments that focus on the model itself?

The West has produced a milleniums-long model of marriage that has allowed for stable Western societies that are based on the procreative family structure, one man and one woman. It has also produced objective injustices around that model (bans on interracial marriage, and no-fault divorce on demand) but over the long haul has managed to hold that definition across many eras of changing mores.  Partnered with full tolerance for other non-marriage cohabitation models, there isn’t any reason to redefine it to remove barriers simply on the basis of making people happy — although Pennsylvania voters are free to change public policy any way they choose.

Had Corbett just stuck to the latter point, he’d have been on firmer ground:

Corbett, a former federal prosecutor and two-term state attorney general, also said he does not think a pending legal challenge to Pennsylvania’s ban on same-sex marriage belongs in federal court.

“The Supreme Court left it up to the states to determine under their laws as to what is and isn’t a marriage,” Corbett said. “The federal court shouldn’t even be involved in this. But if they say they are, then they’re going to make a determination whether the state has the right to determine that a marriage is only between a man and a woman and not between two individuals of the same sex.”

That principle is worth defending in court no matter what one thinks of same-sex marriage.  However, if an analogy has to be used, polygamy is the obvious choice.  If the state can authorize same-sex marriage (and states obviously can), why stop at the two-person model? If the argument is that the state has no business picking and choosing what kind of relationships it should certify as valid marriages and that to do so is discriminatory (as activists usually argue instead in order to shame opponents), then what about plural marriages? After all, some religions represented in the US either tolerate or actively encourage such arrangements, which makes it closer to a First Amendment core issue than a hazy Fourteenth Amendment issue.  Isn’t drawing the line at the two-person model just as discriminatory and arbitrary as drawing it at the heterosexual model?

Don’t expect that argument to win too many debates with the activists, but at least it’s a better political argument to make than relating same-sex marriage to incest — a lesson social conservatives should have learned years ago.