I imagine what they get out of it is being with someone they love. Either they arent jealous or they are willing to work through that in order to be with the person they are with. Being with someone who is poly isnt a deal breaker.
Sometimes the person you are with dont know they are poly and then when they decide they want to practice polyamory the mono person has to decide if it's worth staying with them if the poly person doesnt want to continue being mono. it's kind of a hard spot to be in. someone will be the loser in these situations. either the poly person has to be someone they arent to satisfy the mono partner of the mono partner has to share the partner they thought was going to be only with them.
I think the root of jealousy is often fear. fear they will love another more, fear they will leave you. also there is an element of loneliness if you want to be with your partner all the time and they are out there with others, leaving you alone. also it kind of sucks when your partner is out there doing fun stuff with someone else, stuff that you might want to do. It's kind of on the person having these feelings to deal with them. it's not on me to fix N if he's feeling jealous, it's on him to work through those feeling and vice versa. why would I as a poly person have to fix my jealous mono partner?
Sue, hinge in a vee with Nate (Polysexual) and Sam (monogamous)