Lots of poly people force themselves to be mono, making the compromise for their partner.
You just don't see a lot of them on poly websites and poly support groups, because most of them don't know there's any such thing as poly. They grow up in a monogamous society, expecting that's just "the way to do" a relationship.
Others are cheaters, who have lovers where it's not "just sex" but really emotional. And among those, a lot of the lovers keep waiting and hoping for them to leave their husband/wife, thinking that if they love the lover, it means they don't love their husband/wife. But they love both people, so they carry on the charade.
And then you've got the people who "discover" polyamory and learn that it's an option. Usually when that happens, they realize that they just can't lie to themselves and their partner any longer, they have to follow their heart to be true to themselves. Or, some poly people, like me, feel it's something in them but don't make a big deal of it. If I meet someone else and we have a connection, then cool. But I'm not going out looking for something, because I'm happy with my life and my husband.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker