ok thun you have to understand this...WOMEN ARE DIFFERANT FROM MEN, now that i have said that...yeah pinching your nipple doesnt do anything for you and when your g/f is "stimulated" neither does it do anything for her...now a little anotomy lesson..the breast of a women, when simulated become more reciptive of any contact, wether it be your mouth, hands, vibrator...this stimulation helps to lead to stimulation of other sexual organs, that leads to procreation

in short, dont worry why it does nothing for you (or for her normaly), but she finds it very erotic, just be a good little man whore for your women and she will always take care of you

-my words of wisdom