I might be able to forgive him, but I really don't see myself continuing with the relation anymore. Because where's the guarantee that he wouldn't do it again? Yes, I am aware that people do change and can redeem themselves, but once the trust (which is one of the most, if not the most important and integral part of any relation) is broken, it is difficult to regain in back, especially when you put all your trust and faith into one person and they shatter it to pieces. So yea, if my significant other ever cheats on me, I'm going to end it with him, but I will also try not to hold grudges against him as long as he can refrain himself from doing such a thing again with his future partner.
As for what constitutes cheating - ah, that's the tricky part. Different people seem to have different level of rules and restrictions set for their relationship. As for me, any form of physical contact with another person (baring normal touching, hugging, etc) would constitute as cheating. However, this is not where it ends for me. Any kind of emotional attatchment towards another person that transcends the 'platonic bond' also falls under the category of cheating to me.
Cheating is so common these days that I sort of dread getting into any relationship. But then again, to be fair, it isn't like it didn't exist in the past, just less reported than what it is now. I know I will be flamed for saying this, but why can't people be up front about what they want? Why would you get yourself into a monogamous relationship just to screw your partner behind his back? I understand that temptation is hard to resist - but then, it's not like we live in the 17th century where the option of divorce is not open either.