"When having sex for the second time, she even tried to force me into a relationship or she wont let me do her. "
Seriously? You don't want that girl, she's psycho, out of control and obviously has other expectations than you do.
I can hardly imagine that it would be worth trying to rewire her expectations or being tough on her and hoping she'll "learn the lesson".
Chances are she'll be unhappy and get mad or disappointed or blame you, think you're using her, even if she decides to give in.
Not worth it and not "fair".
To your question, why she's ok with just sex with that other guy.
fators:
- she never had expectations because she understood early in the game he's NOT gonna leave his girl
- they have a different type of relationship, maybe they never went on a traditional date or did boyfriend/grilfriend activities or had the type of conversations that she desires for a real relationship
- maybe she just likes you better (beyond sexuality) because for instance you DID have the type of conversations that make her really fall in love?
- maybe she's just tired of not having a relationship (but won't give up her friend w/ benefits b4 she's got her real b/f) and somehow thought: nows the time for change and he's the one, I'll make him mine!
sometimes you don't even do anything to set expectations.
like there was this guy, who said he wanted to say single, and I also didn't want a relationship, it was clear we were expecting at best friends with benefits, but then I did start to fall in love - which he couldn't handle
happens. But I'd never be a bitch and try to force a guy via sex into something he doesn't want. If you two just don't have the same wants, then that's tough and too bad.