Yeah, I think that is exactly what is going on. Jeanie planted that idea in his head, she is a manipulator and sees this relationship as something she needs to control right away. She is also playing into Zach's masculinity by refering to him in a way that some straight men will from time to time refer to women, thereby cheapening the experence and his self esteme. It is much the same way Gabe views women.
Very interesting! I wish we could have seen more of Gabe at the end, it seems like he just disappeared after the scene in the diner. Finding out your best friend is gay is one thing, finding out he's sleeping with your older brother is another. LOL. That really must've been tough. It's interesting how in that scene, Zach only stopped beating around the bush when Gabe said that it was really really weird that it's with his brother.
Gabe is also there to show how much more mature Zach is, how much more responsibility he feels. And the difference between how Zach deals with his on and off girlfriend, although he leads her on, he still treats her fairly well, and really does love her.
Oh, by the way, I was thinking that there's a big difference between Zach at the beginning of the movie, almost still a teenager on his skateboard, his whole body language is that of a teenager, and then at the end with Shaun and Cody at the beach he really is a grown up man.
End of rambling... LOL!