" You mean it's okay sometimes if we want to enslave people or be racists, sexists, classists, and speciesists?"
If there is no God, then why not? Your ability to get away with it is the only limiting factor.
So logic would seem to dictate, but if you'll notice, these people have certain "sins" (racism, sexism, speciesism, oppression, etc.) which no one ever has the right to commit (at least not against those arbitrarily defined as "the other"). Irrationally, these "rationalists" carry the non-existence of G-d to its logical conclusion with regard to sexual morality, but never with regard to "social justice." I wonder if they've even noticed this about themselves?