I see willpower mechanic similar to fitness. If you do specific action you gain/lose willpower. And it should be harder to gain/lose it Unlike fitnes stat
I found out thats feeding by tube session cant be "forced" by Girl feeder. There is no resist check you simply deny it . Also how about some kinda willpower or Gluttony stat? Thats will main part of all resist checks .maybe 75 of willpower and 25 of girl relationships.