In Miracles, C.S. Lewis makes the intriguing argument that it takes a theistic worldview to explain the existence of dirty jokes. the Christian knows that we are body and spirit, but the materialist argues that we are only matter. But then the dirty (or coarse) joke presents him with a problem; it shows there is something more than just the material here:

“The coarse joke proclaims that we have here an animal which finds its own animality either objectionable or funny. Unless there had been a quarrel between the spirit and the organism I do not see how this could be: it is the very mark of the two not being ‘at home’ together. But it is very difficult to imagine such a state of affairs as original — to suppose a creature which from the very first was half shocked and half tickled to death at the mere fact of being the creature it is. I do not perceive that dogs see anything funny about being dogs…

Another wrinkle: if you observe the animal world, reproduction is a rather mundane affair: animals certainly don’t get bashful or embarrassed about it. But we humans, with juvenile smirks and double entendre jokes have always treated it as something out of the ordinary.

But why?

Evolutionists don’t have any rationale for this different treatment. Are we supposed to believe that dirty jokes help perpetuate the species? There is no natural reason to treat sex as anything other than routine. And if there is no reason to see it as something special, then there is no reason to tell dirty jokes about it. We don’t tell jokes about common ordinary events.

The Christian rationale for this different treatment is much clearer. Reproduction is something special because God has set it apart from normal human activity and guarded it with rules and requirements. And even while society ignores those rules they still can’t help but recognize that reproduction is something special. They don’t want to honor the rules God has set out, but they can’t help but acknowledge His rules when they set out to mock them with dirty jokes.