I'm going to be debating homosexuality in an MUN conference.

I'm posting this to atheism because it will obviously turn into a religious debate. We already had a mock debate with my team and most of the members said it shouldn't be acceptable because of religious beliefs. It angers me that a majority believes that homosexuality should be illegal simply because their religion tells them it's wrong. What good arguments can I use against them? The debate will be if gay marriage should be legalized, so it concerns governments and not religions, but I'm sure it will be brought up constantly. I always see posts with similar questions, so hopefully reddit can give me some good arguments/trolls I can use there!