Why would you leave him if you can be with someone else without leaving him?
I just really don't get the logic there. Sure, if you've got 2 relationships, and one is miserable and not working, and the other is great, then it's going to be a little easier to end the non-functional relationship than if you were mono and ending your only relationship would mean you'd be completely alone and have to go back to dating and all that. But if you've got 2 relationships and they're both good in somewhat different ways, then why would you want to end a relationship that's functioning well? That'd be like amputating a healthy limb just because you've got another one.
If anything, being monogamous means that if you start developing an interest in someone new, then you have to choose between staying with your SO or exploring the possibilities with the new person. I guess the only way the "but you might leave me for someone new!" thing as a reason to stay mono makes sense is if A) there can be only one! and B) you can only develop feelings for someone if you are pursuing the relationship.
A) is countered by any contact with people in poly relationships.
B) is countered by....pretty much any love story where the people didn't start out looking for a relationship, but ended up falling in love anyhow.