Jane, Jane, Jane,
It seems to me like you may have lost sight of the situation just a little bit. First of all, you have to remember that this guy, John, has a girlfriend, Jen- whether or not you consider this girl a bitch is really kind of irrelevant.
The Golden Rule of Dating
Obviously John is somehow still attached to her if he is still with her. There are a few golden rules that I always abide by when it comes to dating and one of them says that you just shouldn't mess with other girls' guys.
Now, it is possible that for John, Jen is someone who has always been there and maybe it is just hard for him to end the relationship- maybe moving to California, as you have said, is his way of escaping her, but the bottom line is that she is still his girlfriend and I think that you need to respect that.
Sex is Not Everything
As far as taking advantage of the time that you have with john I say- of course! Spend as much time with him as you can and enjoy every minute of it- BUT sex should not be the be all and end all of this time- If you have not had sex with him, maybe it is a better idea not to
- If you are a virgin and considering losing your virginity, you should also consider the fact that this is an attachment that is very very strong both emotionally and physically.
- If you are already sexually active then obviously the circumstances are different but I would still advise you not to sleep with him just because and especially since he has a girlfriend- regardless of how their relationship is right now.
NOW, if in fact John is breaking up with Jen, then I say you have the right to do whatever you want to do with him. If you think that the next best thing is sex, then by all means, go for it.
Personally, I think that you are worrying too much about sexual/physical pleasure rather than enjoying the time spent with john. Remember, you can enjoy the time you spend with him without getting in each others' pants.
So, do I think you should take advantage of the time you have left with john? Absolutely!
Do I think that this means having sex with him? Absolutely not.
It all comes down to your judgment of the situation and whether or not you feel that sex truly is the right way to "take advantage" of your time with John. Also, keep in mind that in this case you are the "other" girl, how would you feel if there was an "other" girl in your boyfriend's life?
Put yourself in Jen's shoes before you act on your impulses. By simply taking the step and the time to ask advice, it sounds to me like you are a smart girl and will make the right decision ultimately.