Hi there!
My name is Pretty Woman. Ok not my real name but nice to meet you.

I was in a same sex relationship with my BPD ex. I am now in a very committed relationship with a man. I can't even consider myself bi-sexual. I am one of those rarities... .
I was attracted to the person not the gender. Now I realize I was attracted to a mentally ill person which alerted me to my own issues I needed to fix. Clearly something was off with me if I was attracted to someone like this.
Many BPD's are bi. It gives them more options, more supply.
The longer I knew my ex the more I felt convinced she wasn't gay. I am not condemning the gay community. Obviously I was a part of it and I still have some wonderful friends I met during that time that are accepting of my being straight and in a straight relationship.
Cheating and leaving someone is simply cheating and leaving... .regardless of sexual orientation. Please do not try to over analyze this. She is an a-hole. I am sorry, she is your ex but she is deceitful and you my dear deserve better.
I was very honest with my BF that I was in a same sex relationship before him. I did not want to start anything on a lie. I really like him and he deserves more than that.
Try to separate the whole bi thing from the cheating/leaving thing. She didn't leave you for a woman because you are any less of a man. She left because she is incapable of staying. Her disorder won't allow her to. Being bi honestly just gives her more options and when you are BPD you need options because of all the bridges you burn. I hope this helps!
PW