I think I might understand this....I have a friend who has occasionally been a fuckbuddy in the past. He had a really difficult time staying hard and the 3times we tried, he always required a substantial warm up, a few minutes of sex, then a money-shot finish on the tits or face. It seemed like when he was in all the way, he would go soft. I used to attribute it to his pot habit, but I think that it's psychological. Last time we "hung out" we didn't have sex mainly because I realized after a few minutes of handjob that the problem was still there (10 years later). He did mention that he actually rarely has sex with his girlfriends (when he his dating) which makes me think that it is more than just anxiety at being with someone different.
It's a shame too...he's super-hot, 6'4" and a lean 270lbs