I think cheating only becomes desirable when you're lacking something in the relationship you're in already. That's not to say that you don't love your girlfriend, but it does probably mean that even though she's close to everything you ever want and dream of, she's not THE one.
I was with my first boyfriend for 3 years. For the first 2.5 years I never even looked at other guys never mind thought about cheating on my bf with one. Then something changed in our relationship. We still loved each other, we still cared for each other and as far as everyone else was concerned we were still perfect for each other. But in the end something just wasn't right, even though I couldn't put my finger on it at the time (or even now 4 years on!). For all intents and purposes he could have been THE one, but I think if I had made him THE one I would have been settling in some way. I think that's probably where your urge to cheat comes from too. There's probably a part of you thinking...is this the best I can do? Is she really the one for me? If she really is your first, then you don't really have any experiences that could prove to you that she is. You might just think about cheating, you might not actually do it, but the fact you are thinking about it probably means you think you're settling in some way with the girl you are with...and maybe you are. Think about it...