People who support traditional marriage often say gay marriage will lead to polygamy. This fails as an argument not because it’s false, but because it will not persuade anyone who does not already agree. Sure, a gay marriage supporter may dismiss polygamy as a slippery-slope argument, or say that no one is calling for that, or may simply deny that it will lead to polygamy; but those replies are only for appearances. There are essentially zero supporters of gay marriage who have any objection to polygamy or anything else, as long as it only involves consenting adults. Now, polygamy may not become widespread enough that poly rights activists demand the right to marry, or the state may refuse to let people with multiple partners have insurance benefits, but that won’t be out of respect for the sanctity of marriage.
Contemporary civil marriage is like Poland; it doesn’t have a defensible border. No-fault divorce was the end of civil marriage as an institution. Gay marriage is just the enemy forces occupying the capital. Over the last hundred years marriage as understood in the US has changed so much that it has ceased to mean anything, except religiously or personally. People for years have been able to get married, or not, and stay married, or not, or cohabit with any person or people who will have them; it makes no significant social or legal difference. The only lifestyle choice that draws a hostile eye is non-Muslim religious fundamentalists practicing polygamy, and that’s only because people suspect minors are being coerced.
UPDATE: A point I haven’t heard elsewhere, from 3 Non-religious Reasons to Oppose Gay Marriage
“Not too long ago, we were told (about once ever 20 minutes or so here in the Bay Area – YMMV) that marriage was just a piece of paper at best, and at worst was a the perpetuation of misogynistic patriarchal hegemony, nothing more than institutionalized rape. Homosexuals, in particular, made constant hay over the stupidity of ‘breeders’, specifically and pointedly mocking those of us married with children. We got the point: Marriage, in the opinion of vocal homosexuals and other enlightened thinkers, was stupid and vile, and not getting married was a primary *positive* feature of the gay lifestyle.
Well? Where is that Greek chorus now?