This sucks, but if she left you for something this stupid she's probably a bigot anyway and not worth it.
Some people I am sure just try to make themselves hate it too, they probably just overreact and then go convince themselves that you're sick and disgusting or that you might cheat on them with anyone, that still makes no sense.
The world fails
I decided to check a GSA forum for advice since I knew this topic was there and there where a few people who where straight that said they wouldn't care if their parter was bi and a few people who where like you and got dumped for it, but after about a week they where happy again.
Also strangely enough there where a few people who where bi who said that when they where in same-sex relationships their partner disliked their bisexuality.