In the in-game menu, go into Status then Relationships. There you see your progress with each woman. Pay attention to the colour of the hearts boxes, some are blue while some are pink. Blue ones are for passion while pink ones are for lust. Your passion grows each time that you fullfil a relationship with a woman of passion, meaning when you unlock all hearts from a woman whose hearts boxes are blue. If you fullfil your relationship with a woman whose hearts boxes are pink, then it is lust that you grow. Got it? :-)