Hey sorry I never responded to this topic, I'm honestly not sure if there was a point at which only sex which could result in pregnancy would remove virginity, but if that was the case it was always meant to be updated to any and all sex.
I'm mentioning this now because this gives me an idea for something to add to the game. Since Monster inventory has been added to the game, and Monsters have equippable items they can use, it wouldn't be too hard to add a "chastity cage" type item which would just prevent a Monster from engaging in any kind of sexual activity.