I'm not necessarily the best placed person to mod this community, but I wanted a place to talk about the broader world of ENM, and not just polyamory. (No shade on my poly friends out there, though!)
Eh, ethical non-monogamy is a bigger tent which includes polyamory (people who have loving relationships with multiple people) and others who are more casual about things for one reason or another. Maybe they just have friends-with-benefits relationships as well as a more established partner, or they like to have threesomes or group sex, or something else.
Basically anybody who isn't monogomous is non-monogomous!