Just some thoughts...
NFs approach love, perhaps as a search for a person. They commit to the person, in a relationship.
This means they could slip in and out of actual relationships, without ever really cheating. Since the love for the person may have been very real. So to them, it isn't a betrayal at all. You love different people in different ways. Whether or not it was in a relationship is secondary. And F as a judgement filter, means personal shades of gray as to what constitutes faith, and what constitutes betrayal, isn't it. I tend to think of Fs as reactionary that way... i.e. if they were tempted and circumstances were favourable, they'd see it as another love, and separate and distinct from their other relationships. Not sure if that is clear..
I think NTs tend to commit to the relationship, and not necessarily the person. In that, they choose someone, and even as the person changes, they tend to be blindsided in their commitment and stay on even when the feelings have died. They're committed to the commitment. Betrayal is the breaking of the faith, whether or not the other person was hurt is not the question.
Both approaches have their flaws obviously. The F approach makes me nervous, I'd have to admit. It makes me wonder when love dies, would the person just walk. i.e. he sees no reason to be committed anymore. Whereas a T would keep their word, miserably, until an external catalyst comes to change the principle. I hope I'm wrong. But I don't know.
Hard questions...