I tried living mono for a few years. I did it successfully insofar as there was no cheating, unsuccessfully because I was miserable and, as a result, so was he.
After many talks we agreed to open the relationship on my end. We felt much better right away, even though it was about two more years before I actually met someone. Those two years living monogamously de facto rather than by force weren't hard at all.
I think the people who can go either way are more likely to think they're mono if they are with a mono, and only realise they could go either way if they're with a poly person.
Also, I think restricting what you're personally allowed to do and what someone else does is different. That is, I wish my mono partner had other partners, but he doesn't, and I have to accept that. He would find it easier if I had nobody else, but I do, and he has to accept that.
Forcing yourself to have nobody else seems to me more similar to someone forcing themselves to have more partners when it's not for them, if I am making sense here.
Mono people who "live poly" don't do it on their end of the relationship is what I mean. Doesn't mean every mono will be able to have a poly relationship or that it isn't hard for them, but a poly relationship can be so on one end only while a mono relationship is mono for both partners, by definition.