At the risk of sounding a callous bastard, i would agree, just forget her. Ive done the whole GF thing, the fuck buddy thing and neither work for me. If a girl can keep me interested long enough for me to accidentally walk into a relationship then I tend to resent it and miss being single (Im not callous enough to actually cheat though, just to clarify). The fuck buddy thing doesnt work in my experience, because they either get too attached and associate too much meaning to what you're doing or they make you feel 3 feet tall. So yeah, my advice is; forget her and move on. You might like her now, but tbh if you're not absolutely obsessed with her then you might regret it, especially if you want to just stay mates. You yourself said that you've had better, personality wise as well as looks, so you'll soon find someone else that should interest you, you're just feeling territorial jealousy atm.
Edit: Wuvs speaks the truth. You might hate it Hurp but for me at least its true. He doesnt owe her anything, he hasnt spoken to her in a year or something, so he's not hurting anyone by sleeping with someone else