Perhaps because many communities deeply believe sex is more than just for pleasure?
I'm suggesting that it can be valued for pleasure, and it can be valued for procreation. Because both are true (at least in the PIV scenario).
The anti-sex folks are following old, overly simplistic, and ineffective campaigns to "protect people from themselves (via their genitals)". Most of it is well intended but oh so wrong.
Edit: the only problem is the boundaries of power -- wanting to apply religious beliefs to the whole population rather than their particular sect. And I'd wager a fair amount of the conflict over this power is manufactured in order to keep the base riled up.