What do gay folk and polygamists have in common?

It's not a rhetorical question, I genuinely don't know. Let's see if Megan can help.

The ban on gay marriage is sustained not by solemn policy arguments, since there is no actual hard evidence on either side. It's a social taboo that rests on Burkean principles: no society we know of has ever had gay marriage, which maybe ought to tell us something. The legal ban on interracial marriage was a local phenomenon in the South, and the laws were invalidated by a court with a northern majority. Once you have established that society's ideas about what constitutes a valid marriage are not a relevant consideration, I find it hard to see how you can forbid a marriage just because one of the partners happens to also be married to someone else.

I'm trying to be charitable and presume Megan is going to end up in favor of gay marriage, somehow, but that's a very bad beginning. This

no society we know of has ever had gay marriage, which maybe ought to tell us something

stands out especially, because it's so fucking wrong.

The law of averages being what it is, we are probably not the first culture to ever think of the idea. So if it isn't around, this suggests that societies which tried it either didn't survive, or abandoned the practice.

The aboriginal peoples of North America all but died out cuz of gay marriage. You heard it here first.
Next, the blogger Megan is responding to says

Plural unions have historically most often taken the form of one man having many wives. It seems likely in practice it would take that form in the future. This raises many concerns different from those raised by same-sex marriage, including the greater potential for abuse of women and children.

I want to call this a good point, but I'm not sure we can presume same sex families would prove immune from continuing cycles of abuse and violence. Those are related to the family histories of the people involved, not their sexual orientation. Still, Megan's response is fucking stupid.

Huh? How does having more than one wife make a guy more likely to beat his kids? To be sure, polygamy tends to be embedded in societies that tolerate more wife beating. But the polygamy is not the cause of the beating. To make this assertion stick, you'd have to have some evidence that abusive husbands are more likely than others to take more than one wife.

You mean a man likely to see women as chattel is more likely to beat them, too? Bullshit. If Megan doesn't see it, it ain't there.

On first glance, the argument seems kind of plausible: husbands who come from cultures that tolerate spousal abuse will be more likely to engage in polygamous marriage. But think about this. The women in abusive marriages to those men are almost certainly going to be from the same culture, the children of conservative parents. They wouldn't be allowed to marry outsiders anyway; plural or single, they'll end up wed to someone who might have been raised to think its okay to slap your wife around once in a while.

They're gonna get beat no matter what, so let's not blame the mechanisms used to keep these women in effective bondage for enabling the beatings, sheesh.
Then comes a long bit on the respective impacts of gay marriage and polygamy on the... marriage opportunity market and other technical quibbling we can skip.
Megan closes with the following

Ultimately, I think the gay marriage debate made us ask "What is marriage for?" And the answer we came up with is "Dunno, whatever you want, I guess." Having said that, I don't really see grounds on which we can ultimately deny polygamous couples groups the same right.

Let me be clear that this is not some backdoor argument against gay marriage. I frankly don't see why legal polygamy should be any worse than gay marriage. Which is good, because I'm pretty sure we'll see it within the next few decades.

So gay marriage is unknown in human history and probably for good reason, but she's still in favor of it, like she's in favor of letting junkies shoot up, I guess. Polygamy shouldn't be judged by the fact that it tends to be a key part of cultures that are very abusive of women when it exists in reality, but rather in some abstract sense where some swingers want a permanent threesome with legal benefits. Same thing as two men or women in a deeply committed relationship, absolutely.
It's not man-dog love that gay marriage will end up enabling, just polygamy. Then comes man-dog. And, eventually, manbearpig.