Legal marriage has no reasonable link to gender. It's not about reproduction. Nobody has you sign an affidavit swearing to your intent to reproduce before you can get married. Nobody has proposed that we outlaw marriages involving barren or post-menopausal women or infertile men. Legal marriage is simply an option pairs are given to enter into a certain set of legal rights and obligations, to accomodate a kind of peer relationship where they share a household and a desire to act as a legal unit for a number of purposes (insurability, healthcare proxy, free inheritance, and so on).

There are countless reasons it would be difficult to impossible, and very likely unwise, to try to expand legal marriage to encompass plural marriages or marriages involving minors. For example, just think of the hundreds of thousands or even millions of lines of legal code and precedent that would need to be rewritten to accomodate three-person legal marriages -- pretty much the whole tax code, nearly everything dealing with government benefits, and every major case dealing with spousal rights and obligations (for example, you are in a coma and one of your wives wants a certain treatment for you and the other wants a different treatment -- who prevails?). By comparison, legalizing gay marriage is as simple as legalizing interracial marriage was -- you simply alter a couple lines of text and create a rule that voids anything that discriminates against those marriages.

So, the slippery-slope paranoia notwithstanding, we're not going to be legalizing polygamy simply because we've legalized gay marriage. As shown in places like Massachusetts, legalizing gay marriage is a simple act that merely requires striking some prejudiced language from the law. Legalizing polygamous marriage would require the top to bottom reworking of our entire marriage system in a way that would fundamentally alter the rights of everybody and have untold repercussions throughout the system.

But I suspect the anti-gay-marriage crowd doesn't REALLY think legalizing gay marriage would naturally lead to legalizing polygamy. They just don't have any decent arguments against gay marriage so they want to change the topic to an argument they think they can win.