Guys of reddit: How would you feel if your girlfriend told you she wanted to have sex with a girl just once (without you)?

I'm in a monogamous heterosexual relationship with a cool guy, we've been dating for under 2 years and I am just REALLY interested in having sex with a girl. I've never done it before and I'm not interested in having a relationship with a girl at all, I am just extremely turned on by girls and lesbian porn and feel a huge need to have sex with one without ruining my current relationship.