According to Bridget Jones, you shouldn't stay with someone if you can name at least 3 reasons as to why you should break up.
Number 1) You cheated. This should be a deal breaker, but seeing as you didn't actually cheat with physical intimacy, I can see why this would be a questionable way to phrase what happened. Dishonesty is lying. And lying is cheating. And I caught you. I witnessed it in all of its seedy glory. B says that what you did was a form of voyeurism. You may not have touched, you may not even have made contact -- but that's what makes it dirtier. Plus you kept it from me and had no intention whatsoever of telling me had you gotten away with it.
Number 2) You make me feel like I'm second rate. I'm always going to think I am second best (or even third and possibly fourth!!!!) to your first love, the love of your life, and your one and only friend with benefits. I was denied even those titles! If and when we do decide to call it quits, I will be just another red on your ledger. I am your nothing special ex girlfriend who had to fight those demons, those shades and shadows of your pas,t in silence.
Oh God, I've become one of those girls I detest so much, haven't I? Someone hand me a Valium. I need sleep.