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.
I’m not supporting it, but how is it inconsistent?
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)
If God wanted people to only have sex for reproduction why didn't he design the body so that sex was only for reproduction?