Because! What if I have to buy nice (read expensive) wedding presents for my gay friends? You know they usually have taste, and it would be expensive.
I think in a few more years we will laugh about how crazy backwards people were against same sex marriage- just like how we laugh how people use to be against interracial marriage (and TV characters and contestants couldn't date the opposite race without a bunch of controversy.) Incidentally, they use the same religious arguments against interracial marriage as they do today against same sex marriage.