Sure about sexual identity, unsure about the next step to make?

I figured out I was gay when I was about thirteen. Ever since then I've never really doubted it. I've always been attracted to women and been in relationships with them. I identify 100% as a lesbian. My girlfriend and I are in a pretty serious relationship. I love her and I'm pretty sure I want to be with her for a very long time, possibly the rest of my life.
However lately I've been feeling very confused about one of my friendships. I met him a year ago, during a time when my girlfriend and I were broken up. I had a big crush on him but never anticipated anything happening because a) I'm not really the one to jump on guys and b) he had a girlfriend. Fast forward to now. He's single and he texts me all the time and very obviously flirts with me. Just last night he fessed up to having a crush on me which launched me into a state of confusion. Ever since, I can't stop thinking about what it would be like to be with him for just one time. I've never ever slept with a man but I've always been curious and I think out of all the people I could sleep with, he would be the best option. He's very trustworthy and I'm not afraid of him catching feelings because he's not that type of guy. Basically I just want to have sex with him one time because I want to know what it's like.
But where does that leave me? If I do, I'm a terrible person for cheating on my girlfriend even though it would strictly be just fucking and no feelings. She has also cheated on me in the past. But if I don't, I'll spend the rest of forever wondering what it's like. I'm just so confused, need someone to talk me through this and be real with me. I feel like my head isn't on straight.