I guess I see what you mean, but the fact that I have always felt polyamorous even without practicing it, even when completely single for instance, leads me to believe it's about more than what you do, it's also about the way you feel.
I don't feel that someone in a poly relationship is necessarily poly. That relationship is probably not their first choice, ideally they would want it monogamous, but first of all they want their partner to be happy, so when they can go past the fact that they work differently, it might start to work.
When I first came out to my husband about being poly, I thought that everyone was poly, some people were just kind of formated to frustrate themselves, or something. So at first I tried to make my husband feel that he was completely free to see other people. But now I have realise he can't, it's not him, it doesn't work for him, he can't conceive it.
While he has to make a lot of concession for me to be able to remain polyamorous, I realised I have to make some too. I realised that I had been feeling extremely pressures and stressed out due to his getting all of his emotional and physical needs from me only, like I had to perform 100% of the time, to always be there, to always want him when he wanted me or he'd be frustrated, etc.
I realised was hoping he'd be poly so I wouldn't feel as pressured. But that was wishful thinking. That's not how it works. Just like he has to work on not feeling jealous and realising I do love him and am not planning on leaving him for someone else, I have to work on not feeling pressured and realising that he's not going to resent me if I'm not in the mood.
After all, it's not like when he's not in the mood I go see someone else instead, so I don't know why I felt like that to begin with.
Anyway, to me a mono/poly relationship is poly, but the mono partner is still mono, and I don't think it's him being poly but holding out. In my opinion, while there are probably more polys that one could be led to believe from the societal norm, there are definitely monogamous people as well.
I feel it's important to trust these people when they say "I'm mono" and not try and convince them they're polys on the wrong track or something, because I wouldn't want them to tell me I'm a mono who's lost her tracks, either.