My definition of cheating is kind of harsh.. I heard it somewhere and it fits with how I feel about it..
Anything you wouldn't do with your significant other right there, is cheating.. If you wouldn't flirt with someone with your SO right there, then that's cheating.
Obviously that is harsh, and it's not something that's easy to follow. We are human, we need attention.. It's especially hard when being in a long distance relationship which I'm currently in. I do however live by this, and my fiance does the same. We trust each other 200%, and it's a great feeling.
No, I would not be able to forgive someone who cheated on me and had sex with someone else. Any kind of intimate relationship, physical or emotional, would be beyond of what I'd be able to forgive. We teach people who to treat us, and forgiving something like that is setting yourself up for heart ache.
I respect other peoples definitions and forgiveness, this is just how I feel about it..