Even if your motivation is ultimately selfish, karma does work. Fucking a guy's girlfriend that you don't know or barely know is far less cruel than fucking the girlfriend of a friend or best friend. I myself have abstained from it many time and never have betrayed a close friend in that way.
I sleep well at night. If you're a sociopath, you don't worry about his feelings, but you still might be afraid of retribution. Psychopaths still have a drive for self-preservation.
I reap some karma, but not too much. I've been punished for my sins with onslaughts of jealousy and something that I tangle with to this day. It's slowly dying but not quite dead.
As for the "ultimate goodness" of an act, I can't make that claim. Goodness always comes back to it's consequences and is something the Buddha never strayed from. He never saw goodness as something in and of itself. He always pointed to their consequences, so that's the viewpoint I take.
I think that, practically speaking, you made the right call and found out something dark and hard to accept about women, but something you MUST come to terms with. The locus point of female morality is VERY different from male morality and she seemed to have guessed that the benefits of taking your cock trumped the benefits of staying loyal to him. When you decided not to act on her desire, she went into auto-rejection and became disgusted with your virtue. I'd need to know more about her, but this seems like an innate female drive (though more active in some women than others).
You're not a bitch, but she did think you were. The question is, does that matter?
It might. It might not.
Want to make her wet? I'll show you how.