Pleasure. Strengthening a relationship. Sex for such suspect reasons was sin.
Few Christians today would agree with that. What was so obvious in one century eventually became anything but obvious in another.
Social conservatives like to think that they are holding to something timeless. In fact, they are holding onto some notion of normal they learned when they were first forming their opinions about the world. It's true that what they believe falls into the category of "how it has always been" but that "always" simply describes all they have ever personally known. Time doesn't even leave continents in one place, much less notions of normal.
History changes everything and you can see why this unsettling fact would lead so many social conservatives to reject the notion of evolution. Once you admit that things have continually changed, you have to admit that there's no reason your generation will be spared from - or ruined by - change.