Dude, this game is amazing just 'cause we have so many girls and you can't have them all with the same attitude.
Jill is not Sage. Jill isn't like Quinn. Jill is not even like Maya or Josy. She is a shy girl, from a good family, not used to having a relationship at all and she has to really believe that you're a good guy for give yourself a chance. I think that she never allow the MC to have a relationship with her if he bangs other girls. And I don't think that she will give him the possibility to have sex without a relationship beetween them.
And it makes sense: if you want Jill, you have to respect her and be a Chick. Severe but fair.
The same is not true of Maya or Josy, who are much more uninhibited girls and you can stay "neutral", ready to switch DIK in the game.