Kissing or not, if my boyfriend kissed another girl, I would be pissed >.< I classify that as much cheating as having sex.
I cried alone every single night. It felt like every day that passed here stole another piece of my real life away. After i cried, I’d go and fight as hard as I could. My only thought was winning, moving forward, and getting stronger. — Asuna Yuuki
Been in the same situation before. She gave you the chance to show you wanted her, completely ignoring her "boyfriend", and you didn't go for it. Well you did, but only in your "semi" way that you've justified to yourself as being morally acceptable. In for a penny, in for a pound, mate. You can't be doing with half measures in this sort of situation. Sorry.
Alas, I did exactly the same thing and kicked myself for a good while afterwards, so I really do emphasize. Do your best to move on, learn from your mistakes and don't let it happen again. And really, as others have said, if she was able to cheat on her boyfriend with you then you have to consider the fact that she could do the same to you in the future. The trouble is, that isn't something you can see at the time because you're completely blinded by infatuation. You've put her on a pedestal where she can do no wrong, so you're not really looking at the situation critically.
The biggest problem I had (and I am guessing it is the same with you) is that I found a rare connection. I'm not saying I am a catch but I get girls who really like me but I don't feel the same. I also find girls who I really like but they don't care for me. When you find that mutual connection it is a rare thing and you don't want to let it go.
All I will say is if you hold out for her, and your heart will try to ensure you do with all it's strength then cest la vie. Just don't be bitter and resentful if it doesn't go your way which I would suggest most people would agree it won't.