How can atheists constructively talk to Christians about their religions's rejection of homosexuality?

As long as Christians follow the Bible, they will NEVER by and large accept homosexuality. Just read Leviticus....I think thats the one. There will be a few who decide they can ignore those outdated rules, and good for them. Its a wasted effort though, what you are suggesting. The change must come from within, or not at all, they will not listen to atheists.