Nick has been living in the fast lane sexually and has been used to one night stands which he controlled. Now all of a sudden he is in over his head (so to speak!). He has new feelings he can't describe. Is it love? He doesn't trust people (given his history not surprising) and so he misinterprets Jeremy's reasonable requests as a prelude to giving him the flick (not surprising given his low self esteem). Plus he is dominant like Ennis and doesn't like it now that Jeremy's greater maturity is trumping his confidence and sexual experience.
Solution? Go and screw the nearest available bloke to confirm his rotten opinion of himself, distance himself from an emotionally confusing situation and regain the illusiion of control.
On the other hand as Sheriff Roland points out, it could be just that he is " 22 and all horns."
We can always rely on Roland to circumvent the quasi-psychobabble and bring us back to earth!
PS I wonder if this is a classic example of the different approaches of males and females to an issue. We women go into great depth analysing things from every psychological angle.
Whereas the guys say: He just wanted to get laid. What's the big deal?