Poll: Is Being Gay Cheating On Your Straight Partner?
No.
As for the question in the OP itself: Yes, this is cheating. I would feel betrayed and sad if I walked in on my girlfriend with someone else, regardless of gender. At the same time, I can't deny that I think and feel that it would be hot if she was having sex with another woman. People are weird like that.
It's the difference between fantasy and reality, I guess. Whenever there's an article about an attractive female sex offender, you always see thousands of comments with "boy, I wouldn't mind getting molested by her!". Although it shames me to say this, I always think the same thing. Intellectually, I know it's wrong. I know that even with my particular kinks, getting raped or molested by a woman would be quite upsetting.
I just don't fully
understand it. I can't reconcile the facts I know with any simulation I can run in my mind. If I try to picture the heartbreak and sadness of my girlfriend fucking some woman, I get distracted by
my girlfriend fucking some woman. It becomes a fantasy, and it ends up sounding hot. Then again, I would probably be okay with my girlfriend having sex with a woman if she asked me if I was okay with it first. Sounds like a small difference, but it really, really,
really isn't.
So to answer your question, yes. It is cheating to sleep with someone other than your partner unless they've told you they're okay with it.