Going to keep this short.

My understanding of relationships is that the girl will stay if you keep her interest level (determined by sexual attraction) high. When it drops she cheats or leaves.

My situation - Girl leaves but tells me she can't imagine not having sex with me. In fact she goes as far as to say it is going to be pointless for her to go elsewhere as it just wont compare. She says she is jealous as hell at the idea of another girl's hands being on me.

WTF?

I would say it is more like the exact same for you as it is for her, girls are attracted to all kinds of guys, but just because she is attracted doesn't mean she will cheat, some girls just cheat, some don't, the chances of a girl cheating are usually based on the amount and quality of her options and how much she values you as a partner as well as her views on morality

some girls (especially young girls) just like to hookup with a guy until they get bored then they dump him and move on, some girls will just get a bf and cheat on him like crazy, so they can look less slutty (committed to one guy), and do the cheating in secret so they can get there fix of being promiscuous, some girls will just stay committed and monogamous and want to just stay with their guy and marry him and will never cheat in their lifetime, girls love sex just like we do, it really just depends on the individual girl