I've been with my man now for a little over three years. We've spent 4 months of that time in a long distance relationship, during which I found out that he had gone to his ex's house to see her, and ended up watching a movie with her. I was heartbroken at the time, but then I started to think... he denies up and down that he did anything with her, his friends all say he's the type who would never cheat (even his friends who had been my friends first), and he's given me no other indications of such. I need to just trust him. And our relationship grew stronger because I learned better how to trust him. We have both been to strip clubs, I know he watches porn occasionally, and when he goes out with "the boys" I know he occasionally talks to girls. Why do I know this? Not because I'm dressing up like someone else to spy on him, not because I'm having someone else spy on him, none of that. I know it because we have the trust thing going on, and he feels like he can tell me those things without me getting all jealous and all up in his face. I would never deny him the right to admire the beauty of other women. I certainly don't wan't him telling me I can't look at a guys butt, smile, beautiful physique, get a silly smile on my face and say to myself, "mmmm, yummy!" I am still a human being, and I still get attracted to other people, as does he. We both just realize that we've found the person we want to be with for the rest of our lives, and we're attracted to each other on a much deeper level than just these little crushes that come and go.
What I would constitute cheating is any kind of physical contact in an other than platonic fashion, ie. cuddling, kissing, sex, anything within that scope. Also, a friend of the other sex with whom he could tell things to that he didn't feel he could tell me, or whom he spent excessive amounts of time with. That would be cheating. Cuddling, kissing, and the friend thing are forgivable for me, on one offense, and possibly the second. Sex, on the other hand would take a great deal to forgive, and I'm not sure I would be able to. But I believe in the power of honesty and trust in a relationship, and I believe that trust is a living, breathing kind of thing - it can be broken, but it can also be restored. Therefore, I have no strict lines as to what is an "unforgivable offense."