Leonardo said:

it's from the movie, "South Park: Bigger, Longer and Uncut"

Mr. Garrison: "Well, your moms are just upset. Theyr'e probably all on their periods or something." Gregory: "Mr. Garrison, Wendy and I think that was a sexist statement." Mr. Garrison: "Well, I'm sorry, Wendy. But I just don't trust anything that bleeds for five days and doesn't die."

Wow, I even have that on VHS. Of course I haven't watched it since I first bought it (not that I didn't enjoy it, just sort of forgot I have it) and completely forgot about that exchange.