To summarize my too long and too redundant post:
We can choose whether or not to allow polygamy, the questions as it always really has been with SSM is whether we want to, based on things like safety, abuse, morality, etc. So far the tides aren't in the polygamists favor, although they seem to have strong (faux) support from the anti-SSM crowd. Polygamy and bestiality seem to be the issue of the last few years that conservatives want to push for ... which seems odd.
First of all, I've never been opposed to same sex marriage. My argument has always been against having to get a license from the government in the first place. Who you're married to should be between you, your spouse(s) and your church (if you're religious). The state has no business in it at all.
Secondly, there is nothing inherently unsafe or abusive about polygamous marriage. That sort of thought is as silly as the morality and "danger to the children" arguments against same-sex marriage.
Lastly, did any state anywhere issues a same-sex marriage certificate before DOMA laws were passed? No. DOMA was passed in the wake of a Hawaii supreme court case that opened the door to same sex marriage there. So, to claim that DOMA changed the definition of marriage is a grossly misleading take on the subject.