Does anyone think this is a good idea?

I have been with my boyfriend for about two and a half years. I love him very much. However, there is a very good friend of mine that is a girl. She is gorgeous. She has dark eyes, dark hair, and olive skin. Her name is Bess.
Bess is a lesbian. Recently, she told me she likes me and wants to be with me. I told my boyfriend, and he got very angry. This is sort of hypocritical of him since he told me about two years ago that he likes guys. Apparently, he doesn't anymore. Still, I am not a lesbian. I don't even consider myself bisexual, however… I know beauty when I see it.
My boyfriend and I have an open relationship, but I'm not sure if I should have sex with Bess. I want to, for she is beautiful and I know she wants me.
Should I make love to Bess and have a wonderful, sweet, gorgeous girlfriend on the side of my beautiful, sweet, awesome boyfriend, or should I just stick with my amazing boyfriend?