Arguing that relationships are just for reproduction is not just an argument against same sex marriage, it's also against sterile individuals and suggests that after a women reaches menopause then she can't be married anymore. After all, men can have children until we die. And all sex has the innate risk of spreading and STD, that is if one of the participants has an STD. It's quite impossible for one to be spread when neither has the bacteria or virus. And not all evolutionists say that life is just about reproduction, that is like saying that all gravitists think that life is about staying as close to the ground as possible and suggesting that jumping or flying is just wrong. As for comparing sex to murder, here's a hint, as long as the sex is consensual it's fine, murder infringes on another's rights. Consenting sex between two adults (regardless of their biological sex) does not infringe on either participant's rights. And you do have to right to say that it's wrong, just like you have a right to say that women are inferior to men and that black people should go back to Africa. But that doesn't mean that what you are saying is just.
And a mini-rant about political correctness for a moment. Yes, some people take it too far. Nobody has the right to not be offended. However, that was not why it started. It started because there are derogatory terms that are used to describe minority groups. I could list some here, but I don't want to activate the censor. The point was to stop the dehumanization of those groups of people.