Polygamy is indeed a slippery slope fallacy.
If our government cannot define marriage as the union of one man and one woman, it follows that there can be no law against the union of a man and several women.
Again, you ignore the fact that the laws, regulations, and policies are in place for a couple, and would need to be completely rewritten. For instance, when one is in the hospital, one's spouse has the final authority, in the case of one's incapacity, to decide what medical treatments will be allowed. Now, when there are in fact,
two spouses, which one gets to make such decisions?
That is why God made lawyers.
Well, that may be true. However, I think that legalizing polygamy will not be as easy, and would certainly take longer than did same-sex marriage, because of the body of laws that would also need to be changed. Also, I can't say that I see anyone coming forward
wanting polygamy, as they did with same-sex marriage. The only people even talking about it are the opponents of same-sex marriage, as a "slippery slope" argument.
However, should anyone actually come forward, and say they want to practice polygamy, as you can see, your "slippery slope" failed, because most of us who support same-sex marriage don't really give a shit about polygamy.
And, just in case you want to bring it up next, most of us are just as unconcerned about incest. We just really don't care what people do in their private lives.