This question is at once hilarious, insulting, pathetic, and revealing. It only makes sense if you believe in two things:
1. Moral thought and behavior are essentially determined by the fear of punishment;
2. The definition of “morality” regarding sex is different than it is for other human endeavors.
And indeed, young children and emotionally under-developed adults do make moral choices based on the fear of punishment. Organized religion relies on and promotes this style of personality organization: “Do the right thing or God will punish you.”
And what is the right thing regarding sex? Every organized religion has highly specific answers to this question. They mostly involve “God doesn’t want you to do sex this way; God strongly prefers you do sex that way.” And if you disagree with God about your sexual expression, see rule #1.
So here is the adult version of atheist sexual ethics: Do not do unto others as you would not have others do unto you.
(Memo to religious people—does this sound familiar?)
In more contemporary and pragmatic language, atheists operationalize sexual ethics this way:
Dr Klein makes a great case for reason and understanding-based sexual ethics as opposed to the “Don’t have sex for fear of punishment” model. Well worth reading.