As a gay man, I think that same-sex marriage should be legal.
Not sure if there are any legal scholars here, but from what I've read up on this, making gay marriage legal would not effect churches that refuse to marry gay people. My church refuses to marry divorced people, and freedom of religion does ensure that.
Pastors can preach politics from the pulpit, but can lose the church's tax-free status if they endorse a particular candidate (or probably a specific piece of legislation).
Also, as far as polygamy and/or bestiality go, allowing gay marriage would not make those legal. They would have to be included in a bill, passed through the legislative session (which would never happen), or a case would have to go to the Supreme Court in which both gay marriage AND polygamy AND bestiality were concerned, and there would have to be at least 5 of the 9 justices agreeing on each issue, separately (ie: if 5 judges say gay marriage is ok, but only 4 say polygamy is ok, then gay marriage is legal, but not polygamy).