I suppose you would also think it was funny to have a woman who was menstrating change her tampon in a closeup scene as part of a skit.
Why is something that is gross considered to be funny?
People think they can deny the genius of South Park by thinking up gross or profane examples and remarking "would THAT be funny?"
Well, no - not necessarily. What your example lacks is both a joke, and a point. It's not funny necessarily simply because it's profane... South Park works because they first had a point, stripped it naked, and presented it to you without any of the usual polite vagary. That some people short circuit on the language or imagery and miss the point is not ~really~ your fault or theirs... it's just not written ~for~ everyone.
During the height of the stem cell debate, South Park showed Christopher Reeve actually breaking open fetuses and sucking the stem cells out of their neck during the Larry King show. Complain about the harsh imagery all you want, but you can't say that someone who sees that hasn't been forced to confront what fetal stem cell research really is.
Comedy break: You want to talk menstruation?
Imagine an 8 year old finding his mother's used tampon in the trash...
"You shouldn't have done that, he's just a boy, poor little feller"
No larger politic there really... but that cracks me up.
Through the show, Americans who aren't necessarily very political have been shown with remarkable clarity that:
If it was done using one dimensional paper cutouts and there was a story behind it, yes there is a good chance it would be funny.