I have been seeing a guy for a while now. Apart from a couple of one night stands it's my first sexual relationship in a couple of years. The guy and I are having great sex, except for one thing. I don't seem to be able to cum. I have no problem getting it up but I just can't climax. The funny thing is that I cum like a lord when I masturbate but when I'm having sex it just won't happen. The guy I'm with is very cool about it at the moment but I'm worried that eventually it will become a turn off for him.