If a church -- any church -- believes that something which is legal is intrinsically immoral by the teachings of that church, then that church can preach the immorality to its followers. This in no was gives it the right to command others, although it does have the right to try to persuade them.

When something is in itself intrinsically wrong (both unethical AND immoral not matter how you look at it), such as slavery or genocide, then all religions and churches, as well as everyone else, have the obligation to speak against and act against it.

We forget that we are barbarians and our mores are not those of everyone else.