What sins do Christians decide should be illegal, and what sins are not subject to legislation?

I don't involve myself in all that legal stuff, but I would imagine it's because all the other sins you mentioned are common to all humans at birth; in other words, they are innate and universally accepted and expected. Homosexuality is contrary to biology and parents don't want their children growing up thinking that it's ok. I don't think it's just because they find that particular sin appalling. In fact, they should find all sin appalling. It is only a particular group of Christians involved in this fight, so you should not stereotype.