As far as I know, controversy is a matter of opinion. In this respect the opinions abut sex can be controversial (in relationship with each other), but that doesn't make sex itself controversial.
"So why does a sensually stimulating activity that often results in true feelings of happiness generate so much conflict?" I think this question is an altogether different one. At its core, sex is primarily about reproduction, and mates can be scarce resources, hence the competition and conflict. But it's still no controversy.
"Why would God want to deprive us of this happiness if he loves us?" First we need to establish that there is a God. Until then all we can say that people are told in God's name to do something. And some people seem to get the kick out of trying to portray a natural behavior as anything other than natural.
I would argue that extra-marital sex became forbidden in order to ensure the paternity of the children. That is why sex is a more serious offense for women than for men. Males try to ensure that they are investing in the raising of their own biological offspring, and one of the easiest ways is to fend off rivals and enforce monogamy on the females.
The controversy is the difference between what people feel as a natural desire and the dogmatic prohibition of sex.