There's nothing wrong with finding some unattainable person hot. The point is acting on it. RoseK is right. Flirtations are never 100% harmless, but looking at a 'starlet' with twinkling in your naughty bits happens to everyone. Relationships are built on the 99.9% of the time you're not having sex.
I speak from experience. I had an affair with a married woman once. We were in our 20s (and thus more like married 'girl') and good friends. She married a skater dude who she told me was more like her brother, and who would have sex with her every three months. We started hanging out a lot more and eventually got more intimate, but didn't consummate before she separated from him and moved out. It didn't last long since she had more stresses than just the separation. I don't regret it but I'm certainly not proud of it, and it's something I'd never do again. That's the only time I ever stepped outside the usual relationship cycle and don't plan on it again as I'm fairly traditional in that regards.
Hopefully not TMI?