There is simply no reason to use such gratuitous vulgarity for a descriptive tag. I mean, despite the plethora of alternate names for the male genitaelia, some even more common in the context of erotica I might add, the tag is still Penis, as it should be. Why should its female counterpart not receive the same dignity?
Ass should probably be renamed to "butt" as well, but I won't consider that one to be as pressing an issue, as the correct term would really be "buttocks" which is longer and sounds awkward, and the word itself isn't quite as derogatory to begin with.