Nerves also play a role...
I found, before I was in an exclusive relationship, if I was with someone new, even though I might feel randy as hell, things did not always go as they should.
The only explaination I could ever come up with was that is was shyness.
I never found this a problem though, as you don;t need to actually fuck to have a good time, and normally, in the morning or the next time, no trace of 'nervousness' was to be seen. ALso, post-cult tension might play a part, as on some level your brain is saying 'you won;t survive Armagedon is you dip your wick young man'.
Sex is new to you. You should not expect to be Mr Loverman straight away... hell, I was married five and a half years and seperated for six months before I had 'r e a l l y g o o d s e x' for the first time.
Take Mindchild's advice, read up on the subject...
People living in glass paradigms shouldn't throw stones...