I just left a 9-year mono relationship, and recently started to live as poly. Back thern, I got by by denial. Simply distanced myself from anyone I felt attracted to, often without admitting the feelings even to myself.
I believe every mono relationship needs to find ways to deal with attraction to others. At the beginning of my mono relationship, I was terribly jealous a lot of the time. Later, I accepted my partner's attraction to others, since I trusted she'd live by the rules we'd set up. I held my feelings for others hidden, since I knew they made her uncomfortable.
After it ended, I re-evaluated my thoughts, and realised I could've stuck with her no matter what I did with others, hence my change in direction.
But I do not believe anyone is emotionally mono. Feelings can be repressed, and everyone finds their own way of controlling them. I believe every mono must find the way that damages them the least.
In the end, I value honesty too much. I've never seen anyone who Can't let another equally close, I've just seen half-lies on the subject, at best situations where you allow emotional cheating. The idea that you get to decide another's feelings seem a bit freaky to me. In my mono times, I simply wanted realistic rules, so that I could trust my partner. If she promised me things I knew to be impossible, I couldn't trust her other promises.
I don't believe I've changed, or that I was lying to myself in living as mono, that I was really poly the whole time. It's just ways of dealing with reality. I believe it to be a choice. A choice that must be respected, of course, and more of a choice to some people: I can't go back from where I am now.
But I feel that the idea of emotional cheating invites emotional abuse.
It might possibly be an effect of the verb/emotion dualism of the word Love. It's possible to promise to not express romantic love for anyone else, but I don't believe it to be possible to promise not to feel it. That'll just end up flooding someone with guilt, and severely skewing the power balance in a relationship.