Mary bleeding

I loathe South Park, and think that Trey Parker and Matt Stone's pureile, misogynist liberatarian politics come through in every episode that I've watched. But I am wondering about this latest fuss.

Everything I've read about the episode of South Park that is outraging Catholics , focuses on the fact that the virgin Mary menstruates. Now I don't know a lot about Catholic theology, but surely that's not controversial? She was a woman (she's half your complex - where's the whore without a virgin), women tend to menstruate as long as they're in reasonable physical health. There does seem to be a whole 'periods are yucky and we must not besmirch Mary by mentioning that she had one' tone to this whole debate. Did anyone watch it, can anyone tell me what people were so upset about?