From the very first "boyfriend" I had in the 3rd grade (the type where you hold hands and share your lunchables) I've always been in poly-relationships. Always.
The thing about it is, you have to set down guidelines and respect one another's wishes, and if you don't trust your partner in a poly relationship, you wouldn't trust them in a monogamous one either.
Some relationships require you to tell your partner about other partners, and in ours we've always had a deal. If someone new was added to the relationship on either side, the other partner(s) must be aware of it, and "approve" of it. For example, I wanted to date a girl who one of my girlfriends didn't like, so I let it go, because I didn't want to cause jealousy or tension between someone I was already with. She trusted me once I asked about this girl, to not go behind her back, and I would've trusted her or any of my other partners to do the same.
My husband knows about my girlfriend and we've all hung out together, and I was with her first and she knew when I met my hubby. Right now I only have 2 partners, and it will likely stay that way, but my husband trusts me FULLY and knows that if I ever had an interest in someone, he'd know about it before the person I was crushing on did.