Is it adultery to enjoy , even within marriage?

God intended for us to have only to reproduce. The ONLY moral reason to have is to make more Christians, and maintain numerical superiority over atheists, Muslims, and the barbarian hordes of the east. When Jesus describes how to look with lust is to commit adultery in the heart, he does not make any exception for marriage, does he? Even if you are having to reproduce, if that is not the ONLY reason, could it be labeled adultery?