Desire.

Girls have a Desire stat from +2 to -2.

NEW BASIC MOVES
At character creation, describe one girl who wants you (Desire +1), and one girl who doesn't (Desire -1).

When you meet a new girl, give her Desire +0.

When you roll against a girl, add her Desire to your roll (if positive), or or subtract her Desire from your roll (if negative).

When a girl reaches Desire +3, mark +1 XP, and reset her Desire to +0.

When a girl reaches Desire -3, she immediately makes a move, which likely includes "Leaves forever".
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When you romance her, describe how you do it. She gains +1 Desire, time passes, and the girls make a move.

When she catches you, in a lie or with another girl, etc., roll+cool to get out of trouble. On a miss, she loses -1 Desire.

When you seduce her, roll +hot. On a hit she gains +1 Desire. On a 7-9, it costs you, or you get in trouble doing it.

When you fuck her, the options include "she gains +1 Desire".

When you force her, on a 10+ you can choose either "take +1 forward", or "she doesn't lose -1 Desire." Otherwise, she does.​
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You can start a new story using these Desire rules here: http://chyoa.com/story/slut-world/thread/stories-using-variant-rules

You can also start using these Desire moves at any point in an existing story. Give one girl Desire +1, one girl Desire -1, and all other girls Desire 0.

Please make sure to say that you are using the Desire rules (and maybe link to this page) wherever you start using them.