In my opinion both are cheating.
A "cyber affair" or "emotional affair" is just as bad.
What it's really saying is (They would be together) if YOU were not in the way. It's the obligation they feel to stay in their present relationship which "intensifies their romance"
It's the old saying, "right after I got married I met my soul-mate" or wouldn't you know it "my girl's best friend is the woman of my dreams......etc Bad timing and unforuntate circumstances always serve to spice up romance.
However truth be told the only person we can control is ourselves. You have no idea if your lover is imagining you being someone else when you have sex. At some point you either trust what you have is "real" or you walk out to find someone who really wants YOU.
The whole point of being in a "monogamous relationship" is not having to share your mate's (heart and body) with another person. You're suppose to be "committed" to taking care of each other's emotional and physical needs. If either person is not holding up their end of the bargain there is likely to be problems.