Why not just outlaw sex? If looking at sex is illegal they might as well go all the way.
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.
I can speak more to the Catholic view than the evangelical, but if you accept the initial premise that there is a creator God then the rest follows fairly naturally (you can look the ‘the theology of the body’ if you want to read more)