A couple more thoughts I had after hitting "post" about the benefits of poly.
1) Our partners get to know us better than anyone. As such, they can help us see things about ourselves that we never saw before, can help us be things we never thought we could be. Partners help us grow. Obviously this doesn't apply if new relationships are superficial or interchangeable, but I don't think of that as poly, I think of that as fucking around. Not that there's anything necessarily wrong with that, as long as everyone understand what' s going on and is treated with basic human dignity.
2) If your bf is capable of loving more than one person at once, which had better darn well be the case if he wants to be poly, he might fall for someone even if you guys are supposed to be mono, without even meaning to. If monogamy is the only option and he's madly in love with someone else, he might leave you out of desperation, to find out what could be with the other person. It happens all the time to mono couples, that or just plain old affairs. With poly as an option, you don't have to kick one partner out of your heart/life, or live a double life, in order to let in someone new.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.