Is this thread really that complicated?
Sounds like this guy got with a girl hoping she would dump her BF for him. Happens all the time. When the BF came back, this gal wanted her cake and to eat it too. Apparently the attention of one guy isn't enough for her. If she really cared for either, she'd break off with both of them. Alas, that is the paradox of someone who cheats, yes? You can't expect them to act with integrity since they've chosen to put themselves in that situation.
OP (assuming you ever come back) - Just cut contact w/this gal. She may be crazy, she may just be confused, but either way its not going to end well for you.
Next time, don't pin your hopes on someone else's behaviour. Either do what Stung does: take the sex on offer, don't let yourself get emotionally attached (she's not available for that); OR tell her you like her but your own values don't allow cheating. If she's insulted: not your problem and you've dodged a bullet.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh