I've been boning this girl, but now her best friend (male, gay) wants me. What do I do?

[Mincing Meat]

I'm sort of in a relationship with this girl, but I'm not really keen on it. She though is keen on me. So I give her a good boning from time to time, but won't lead her on or say I want things to be serious.
(In fact I'd rather she found a boyfriend who was better suited to her, and dump me completely).

I'm a bisexual guy, but she does not know this (she has never asked).

Anyway her best friend is a 22 year old gay guy who is smoking hot. He has never met me (only seen me on Facebook), and also does not know I am bisexual. But yesterday he sent me a message on Facebook saying he fancies me and wants to meet up!

What do I do?
On the one hand I want to meet him because he is a sexy boy who turns me on. On the other hand it seems somehow (in a way that I can't articulate) to be immoral.
I can certainly see that it would annoy or even upset her if she found out?

What do you think I should do?
esmerelda, she's the one always trying to bring me along to her house. Not the other way round.
So I don't think I'm taking advantage of her - if anything the other way round.
Though you're right that she could probably do better than me