So, now that same sex marriage is legal will the gay rights activist now work on getting Bigamy legalize? What about polygamy? And those who wish to marry in the family? Or was it not really about marriage equality for all,but just for them?
Why don't gay-rights activists fight for poly-rights? Because they are gay activists, not poly-activists! I'm sure there are gay people who feel that poly marriages should be legal, but have their hands full fighting for their own rights. Gaining the right to marry is big, but the struggle for equal rights is far from over. And, as Dannyalcatraz has pointed out, legalizing poly marriage would be a complex undertaking. In comparison, same-sex marriage legalization is cake (from a Christian bakery, no less)!
I personally believe that polygamous marriage should be 100% legal here in the USA, and I support the poly friends that I do have. But I'm not a poly-rights activist, in part because I'm a straight, monogamous dude, and not having the right to marry more than one spouse isn't high on my personal needs list. But I would certainly support a "poly-rights movement" if one ever crops up.
I took the same approach to same-sex marriage. I have long believed that gay people should have the same rights as straight people 100%. But, as a straight guy, not having the right to marry another guy doesn't set me back much. So, I am not a gay-rights activist. But I do support my gay friends and I am very pleased with the recent Supreme Court ruling.
And, gay people are people too, in every respect. Some gay people are racist, some hate on Muslims right along with the Fox News crowd, and some do not support the idea of polyamorous relationships or polygamous marriages.
For poly folks to enjoy equal marriage, there needs to be a poly-rights movement fronted by polyamorous/polygamous folks. Just like the gay-rights movement largely consists of and is led by gay people. Far as I know, the poly community in the USA is very small and not politically organized. But, while I do have poly friends, I'm not in touch with what's going on within the poly community, so I could be wrong.
NOTE: Please be aware that polygamy is not just practiced by a Mormon splinter group in the western USA, and is not characterized by power imbalances and child abuse. Polygamous marriages HAVE been used to create power imbalances, usually in favor of men, but the same can be said about "traditional" marriage. Polygamous marriages have been a part of many cultures throughout the world, both in the present day and in past times all the way back to pre-history. Some structures allow for one man to have multiple wives, others allow for one woman to have multiple husbands, and some allow for multiple wives and multiple husbands. Just the concept of polygamous marriage itself is a complex beast, and to legalize it within the USA might be the right thing to do, but it would be difficult and complex even if 100% of our society would be cool with it, which, we obviously aren't.
Do I hope we get there someday? Yes. Do I judge my monogamous straight and gay friends for not adding poly-rights to their activism? No.
TL;DR: Same-sex marriage and polygamous marriage (which includes bigamy) are not the same issue at all. Related, yes, but not remotely the same.