Bold added by me.
This statement made me think about my responses here. I think it's safe to say that it's a problem, at the very least, if one person feels betrayed by the actions of the other person.
I also hope that when I've expressed my thoughts on this subject that it's clear that I am speaking from the position of my own relationship, and I'm pretty sure I said or agreed with someone who said that the definition of cheating is unique to each relationship. Trust is important to me, so I tend to get overly passionate about betrayal of trust, at times. If I have come across as intolerant, I hope that is only an intolerance of the harm one person can do to another, and not of lifestyle choices that are acceptable within a relationship.