I have never had a Girlfriend for any extended period of time, I bore too easily and like to do my own thing. So it's a tricky one, as I'd be seeing a girl for three months and I'm sure she thinks we were going out but I wouldn't, so I wouldn't call it cheating if I scored some other girl. I do suppose it would be different if I actually met a Girl I really liked and didn't get bored of.
But I doubt I'd ever cheat on a long term Girlfriend, if/when I have one.