Well - my two cents here. I lived up in Humboldt which has a lot of gay people, and know about the many issues discussed here.
1)Cheating is cheating, no matter what gender and with what other gender.
2)Hellz - your gf told you about being bi. For some people, that takes a lot of courage and trust. She may have been able to keep it hidden but she decided to tell you about it.
3)Just like anyone else, she may be attracted to others, but that does not mean she would seriously try and initiate a relationship. Most people do google at others while walking around (particularly at beaches) but that doesn't mean anything if a relationship is stable. Heh - ne of my ex's warned me that I would get dumped as soon as Catherine Zeta-Jones called him back.