We've been dating for a few months but we still haven't had sex. We can kiss for hours, but every time we try, my boyfriend doesn't have an erection or it doesn't last longer than a few minutes. When I masturbate him, he gets hard for a little while and then goes down again. He explains this to me that I'm doing something wrong at some point like doing it too hard etc. and he should have a mood for it. I don't know what to do in this situation. Is it my problem that I don't excite him in any way physically? Or is it some kind of medical problem?