I'm confused as to how she was "forced" into having sex with this guy... unless she was drugged/raped. Unless you have a verbal understanding that being intimate with other people is a deal breaker then cheating on someone is not the absolute worse thing you can do; abusing or attempting to or actually killing someone seems like it tops the list. But, if this is something you can't forgive (and by not forgiving you'll have anger and contempt in your heart), you should probably let it go because there will be a constant reminder and you may treat her differently.

Dude, I'm sorry but one party and she cheats? Drop her and move on, she'll keep cheating on ya bro.

She cheated and lied to you. What redeeming quality is there that would keep you with her? If you were married then there might be a chance because there is already a full commitment established. She is already going behind your back now and will again.

i think she doesn't love you at all. she's just stuck on you coz ur too good. she's afraid of breaking ur heart. and the incident that happened kinda tells you that she doesn't really love you. so break up with her and find someone better. thats the best option.