I'm not going to make a rule against using crotch shots in profile pictures.
The reasons are:
* You won't stumble upon a profile pic like you might stumble upon an avatar pic.
* I think it is useful to allow people to post crotch-shot profile pics so that others can use that to identify what type of person they are.
* We already have a rule against nudity, adult content, etc. Seeing someone just wearing a diaper isn't going to make your head explode.