But since atheists don't admit of any transcendental morality or decency for humanity, that is an inconsistency in their setting limits to sex.
What you are doing is saying that all people must have an absolute divine imposed morality or none at all.
What you will find in reality is that people can impose a code of conduct on themselves based on personal and cultural values that they ascribe to.
A divine moral code is adheared to because the individual abdicates their moral/behavioural standpoint to their god. A personal moral/behavioural code is self actuated.