Around 10% of males supposedly cannot orgasm during intercourse owing to a history of Traumatic Masturbatory Syndrome. This is when a guy lies on his front and stimulates himself by thrusting into a mattress/pillow/the floor. What this does is produce a effective orgasm, but places unnaturally high amount of pressure on the genetals, which can not only cause erection problems in later life but also means they can almost never ejaculate in intercourse. Nobody really talks about it but yeah, supposedly around 10%.
It is apparently curable though, and I think there are a few sites dedicated to this though I don't think I've been on them. Even if this isn't useful to the original poster I'm willing to bet some randomer will think "OMG that's me!".
One of my friends managed to overcome it, and I think it's possible for most people to before it turns out to be too late.