It depends on how you mean your question, and how you look at it.
If you are saying "could Gay Marriage change polygamy" and meaning that it is a possibility because you believe that Gay people are in relationships with more than one person at a time - then no, it won't change polygamy - because being gay, and being in a polygamous relationship are two different, entirely seperate things - to be in a polygamous relationship there is no requirement to be gay, and vice versa.
However, if you are talking about it as two seperate, yet minority, entities - I suppose it could, but you'd end up with the same problems - Christian men who cheat on their wifes and get divorced, telling you that you can't marry the person (in the case of SSM) or the people (in the case of polygamy) you love. Thanks very much Newt!
Thanks for this question - made me think!