Well, the sex laws are ineffective unless people do it in public where you can see them. In other words, people not having enough common sense to do it in the bedroom. Plus there are certain behaviors that account for a majority of STD spreading too.
Hang the pedophiles and rapists but don't make illegal what the majority of adults do despite some overreaching laws.
Knowingly spreading diseases should also be condemned and prosecuted but alas - Kalifornia says it's OK.