Forget that whole "what is really cheating".
If it hurts the relationship, it hurts the relationship, and whether or not you agree that it ought to, if you cared about your partner you'd try and resolve it.
eg. "You seeing your best guy friend hurts our relationship"
Instead of going insane because it's not cheating, you talk it through so its no longer a problem, find some resolution, for example, get your best guy friend and your boyfriend to become good mates.
I mean, when my boyfriend went to a strip club when he and I were 16, I got pretty upset but I couldn't understand why myself. It just hurt, and that was all I knew. He assured me he thought I was beautiful and that strippers are purely body, no personality, and he would not maintain a relationship with them. It sounded so silly, but it was what I wanted reassuring or wanted to hear. Now I don't care if he goes.
That's what Chris and I have. Returns results, rather than endless bickering about whether or not its cheating, or whether it should hurt you or not.
And if that ever stopped, I don't think I could tolerate that.
Oh and infidelity and all that I cannot tolerate.