In my opinion, anything He would not do or anything He would not say in the presence of His Partner is unfaithful to the relationship.
When I first posted this question, I thought my opinion was firm and clear. I'm finding when I think about it more, that there is more grey area than I originally thought and it's more complex than my surface response.
What is NOT cheating to me:
porn, going to a strip club, talking to a girl, talking to an old friend that is female.
What IS cheating w grey areas included:
true sexual affair, true emotional affair, online stuff that someone would have to hide from their partner, someone that became more important than the person's spouse/partner/family.
Hiding a relationship or relationships deliberately from your spouse or partner even if there is no sex involved.