The reason people have a problem with polygamy is because in history, it's always been one guy owning a bunch of women. But, I know people who are in polygamous relationships who do just fine, because it's built on the foundation of trust and equality, and we've kinda moved on from the barbaric idea that women are property (okay some people still believe this but you know, you can't fix everyone). In their case, it's not one person owning a bunch of others, but a few people who all just really love each other. I think a lot of people have a hard time believing that, especially when there's people out there who struggle with having just one partner, especially regarding jealousy and such.
Anyway, I'm actually starting to kinda wish marriage equality had waited until other things had passed in the country. The LGBT movement here was largely started by trans people, and yet we get the short end of the stick, considering that most LGBT hate crimes are committed against trans women, and marriage doesn't fix that for us. In most states, you can still be fired, dropped from your lease, etc. for being gay or trans, and well, when a country gets marriage equality, typically steam for LGBT rights goes way down. In Canada, for example, LGBT organizations started only getting about 2% of the donations that they used to since marriage equality passed, which sucks because it doesn't end at marriage, despite what a lot of people think. There's still so much more to do that wasn't affected by it passing. I'm still happy for the people who can get married anywhere they go, it's just like... it's gonna suck if you go out and get married, and the next day, your employer fires you for it! That's a problem.