I gotta say you really shouldn't use the girl for sex, people seem to like to think that these days we can all have free fun no connections no repercussions sex and to some extent we can as far as the physical problems and social problens go ut most people I know who act like that are actually fairly unhappy about it, they don't form proper relationships and actually really struggle with coming to terms with it.
Either work it out or walk away. But don't use her for sex. That is just cruel to her and yourself. When you are purposefully cruel, you loose a part of yourself. The next cruel act becomes easier. And the next.
bah come on man thats nothing, you got it easy. hell i could forgive that and get on with it, you think you got it bad? psh. try listening to her bang the neighbor on new years eve as you lie helpless upstairs vomiting blood into a trashcan with moderate TB ? now THAT will make you hate life!!! see, lifes not so bad.
Girls (or guys) are a dime a dozen, theres a million others out there so ditch her and find someone you can trust before you get so jaded you can't trust anyone (unfortunately like I am). You will always find another one to love or lust after.
Dump her right now. She obviously has no class if she is going to expose herself to some random over the internet, even though she is seeing someone irl.
Women are everywhere mate. Everyother person you meet is a potential partner, so why get hung up on just one? Especially one that is obviously not worthy of you.
Personally, I would destroy her emotionally forever. I'd manipulate her mind so badly she would never recover. What she did was completely evil and sickening. She deserves some serious punishment.
Go back to her and spend a few months playing with her mind, working her up to a state of complete love for you. When she feels like she cant live without you. Shag her best friend/s, and take pictures. Send them to her at work.
Of course it will happen ad infinitum because guys are the first to be bird-doggin' beaver as soon as they catch the scent. Despite the white-knight crap men try to pull they know deep inside no one can resist the power of the poon.
Anyone that needs to get all emotional and woe is me about it needs to head straight to http://en.wikipedia.org/wiki/Emo_%28slang%29 - and then pull the friggen trigger already.... they're taking up way tooooo much oxygen.