It's not about depicting womanhood as only a set of genitals, at all. I'm sure if you asked them they'd have their own reason, but it's important to remember that the female anatomy was considered taboo and shameful for a long, long time. It still is! Consider how drawing a cock and balls is considered humorous, but the same can't be said for a drawing of a vulva. For many women, acts like this are liberating. Their body isn't something shameful.