Potty jokes are one of the things I say that get me in trouble sometimes. Some of that stuff just pops in my head unbidden and rolls off my tongue. Why is it the same people who howl with laughter when a comedian does that are offended hearing it in conversation? I wish I could stop myself from saying things like that.

I think it is the violation of social taboos that causes the logical incongruence (which is the nerdy way of describing humor). The jokes are generally third person so it is a story about someone else dealing with something gross. Sex jokes are also funny when they really aren't that good; again with the taboos.