Assuming hard or above, you lose 10 willpower per day because of womb of lust, but some days you can lose noticeably more, the game provides no context as to why this is or what the calculation is, other than the obvious relation to current corruption, however it's unclear how much it influences it. It's annoying to end a day at say, 12 willpower knowing you should be able to use a blessed water the next day, only to end up with a forced masturbation scene. I think the extra willpower loss is completely pointless and annoying.
PS - pure belphegor needs balance tuning.