Because in their philosophy sex serves a purpose (procreation and the deepening of the marital union).

Anything that doesn’t support those two aims (contraception, abortion, gay marriage, pornography etc.) is therefore immoral but there’s nothing wrong with sex in its ‘proper’ context.

Their philosophy is extremely inconsistent. In reality it's emotionally driven. Not sure what specific emotions cause one to think sex is wrong except for reproduction, but a lot of the people who push this sort of thing turn out to be closeted gay homophobes.