When theists worry about losing sexual morality, it is because we are theists. Some of the reasons we think the world would be better if we are chaste depend on us preferring God to be happy than to be sad, the world being equal. If God could take some happiness pill, we'd want Him to.

That reason to be chaste would apply less if God were less important than He is. As such, our actions would displease the real God more if we believed God was less important. That means that believing God is less important would cause us to act in ways that we now consider worse, if we acted as would be right if God mattered less.