Oooh this seems very interesting and I'm usually not into this sort of stories.
I would still give the other woman the benefit of the doubt because maybe she didn't know he had another gf, he lied,etc...
Anyways, the first culprit is always your bf/gf. It doesn't matter how attractive or seductive the other party is, stay loyal, bitch. Or you could have handled it in another way, like telling right away you don't love someone anymore and break up in a more genuine way, not cheating behind their backs.