If you are into her and want her to feel pleasure, it shouldn't matter if she's running an "assist" on her orgasm.. many women need to in order to get there. More than half the time when I orgasm during intercourse I assist, and lucky for me I have a boyfriend that loves the fact that I share that moment with him.
Would you rather she not orgasm at all, or just fake it so you can feel like you've done some great deed? Sex shouldnt be about winning or losing, you 'giving her" her orgasm. It should be about you guys SHARING in that pleasure together.
If its really important for you to be the one to deliver the final blow... I really hope you are paying attention to what she is stimulating, how she is stimulating herself, speed, pressure, motions, so that you can mimic those sensations. Because it would be a failed effort for you to decide for her body how it should orgasm and give her only what you THINK it needs to do so.
Comment