My advice.
Re-think why the thought of her having sex with someone else is really bothering you. When you figure that out it will make everything make a lot more sense, and you'll realize that there is someone better for you out there.
It's the fact that she would be "getting off" with someone else. That though kills me to my very soul. I know flesh is flesh, but for her to do that....I don't know..it slays me.