Workouts to strength my knees for sex

Sometimes when I get intimate with my boyfriend I end up on top of him and I basically grind on top of him with our underwear. We did this last weekend but didn't wanted to have sex bc he forgot a condom. Anyways, during this position my knees hurt a lot. What exercises can I do so that it won't hurt or reduce the pain?. I'm a dancer so I'm pretty flexible but my knees tend to be pretty weak.