Marriage is traditionally defined as between a man and a woman, and we're talking something that has been a part of human culture since ages. Changing that idea isn't happening overnight.

Regarding religion, it's a problem because many view it as a sin. Maybe it is, maybe it isn't, I have no idea. People have religious and moral beliefs, and if we are to ignore them always, not just when they don't agree with our views on same-sex marriage, then society would be much worse indeed.

Personally, I have no problem with it because it doesn't affect me in any way, and I don't see how anyone else will be hurt by it.