I dont consider porn watching to be a "behind the back" action. I've done it since whenever, most of my partners have as well. Typically we dont interact into each others porn watching, but are aware and not threatened by it.
Chatting is a different story, along the same lines as camming, tributes and the like...there's a personal connection made on a level. I dont engage in it, and I make it clear from the onset of the relationship I consider it a form of infidelity.
I think there's a somewhat corrupted foundation under a relationship where one deems consumption of an actors/actresses impersonal work for personal enjoyment or masturbatory purposes as infidelity...unless there is some demeaning or illegal aspect of that work that interferes with the relationship itself.