I definately have this problem, and it only gets easier when I am comfortable with the girl I sleep with, which means that for sex with girls I don't really know yet it's problematic. practice and trust is pretty important for me when it comes to sex, which is why I am not that much interested in one night stands.
I guess for me the reason is that when it comes to penetration, I have to stay hard, which puts me under psychological pressure, which is of course not helpful if I want to stay hard. but a good handjob or blowjob is as satisfiying for me, and since I am also pretty good with getting them off with tongue and fingers, nobody has complained yet
Definately nerves. I find it is easier to have sex with a stranger than sex with a friend. Try to at least get oral on the very first date. Then it only gets easier from there.