That's not quite true. I don't really care if two homosexual men want to stand in front of a justice of the peace and be declared a legal couple. It's basically contract law, so whatever floats their boats.
What I object to is forcing Christian churches to accept this arrangement as an actual Christian marriage, which is not, and cannot ever be. Two men cannot be married because, in the Christian church, only a man and a woman may be united in marriage under Christ. The union between husband and wife models Christ's relationship to the Church, and two men (or two women) cannot model that relationship.
Now, the immediate cry is, "But nobody's forcing you!" It's a lie, because everybody can plainly see what the endgame is: forcing Christians to accept homosexual marriages (or "poly" marriages, or whatever other novel degeneracy the culture happens to be cooking up). The liberals absolutely will not stop until all the walls are down. If you think that pederasty is the red line, think again -- the liberals are busy chipping away at that sole remaining bastion as well. (And again, there are elements in the church itself that are driving this process as much as the secular culture -- or more. It is positively Satanic.)