I am poly! And it is super duper!
But there is a lot that goes in to it. Here is my take.
It is very difficult for one person to fulfill every single need you have in a relationship.
Mr. Evvie has big strong arms, a sexy beard, and a huge dick. He can throw me over his shoulder, give me spanks, hold me when I'm sad, show me how to shoot guns, and he has a penis to do sex stuff with. He is a masculine man and I love it!
I can't paint my nails with Mr. Evvie. I can't go dress shopping with him. I can't express how I'm angry at another girl and have him accept it even though I don't have a good reason. He doesn't have boobs to play with. He doesn't have a vagina to play with. He doesn't like to talk about fashion, ponies, or girlie stuff.
I love Mr. Evvie, but he can't provide the need in my life for a romantic connection with a woman. I love women, but they cannot fulfill a need in me for a romantic connection for a man. I know some people do all lesbian/gay poly, but I prefer to have a 3 person M/F/F relationship.
Poly can be very hard, though. Communication is highly important. Mr. Evvie prefers that I get to know a lady before they drop any 'bombs' on me about something serious going on with her that would necessarily have to involve us. I prefer to know everything up-front. This has lead to some issues between us, but that's in the past. Communication can also be difficult to "re-calibrate" when there is an extra person involved. It seems that already established paths of communication remain strong while new ones form more slowly, which can lead to one person being unfairly left out.
NRE ("New Relationship Energy") is another issue that can be hard to work through. When you get a new boyfriend or girlfriend you want to do everything with them because it's an exciting new relationship. But when you already have someone with you, it can be hard to maintain your previous partners. This has lead to situations where, for example, I was not told that a woman was moving in to my home until the day her luggage showed up and it was implied that they wanted me to move out. Later on I found out that both of them assumed the other one told me what was going on and they didn't want me to move out; but in the excitement of that NRE, both partners kind of forgot about me, and because they were so involved with each other, I felt like a trespasser in my own home. That's a pretty extreme example, but some form of it is common.
If anyone wanted to do poly, my question to them would be, "why"? It does take practice to work, just like a regular monogamous relationship does. For a male/female couple, it is also considered extremely difficult to find another bisexual female partner to join them.
I would also like to say that I consider poly relationships to be different from open relationships. In my opinion, "Well yes we're poly, my boyfriend gets to have sex with all the women he wants" is not a poly relationship. A poly relationship is with three or more people who are all faithful to each other. An open relationship is where two people who are dating also get to have sex with whomever they want.
Many problems also seem to arise from males saying, "I get to see women, but you only get to see women, too. I will leave you if you have sex with another man." I've heard several stories of women being so unhappy and upset by this arbitrary restriction that they ended up cheating on their boyfriends. If poly is just an excuse for you to get laid and watch your girlfriend play with another girl, I would say you probably are not in for a happy relationship. If you get to sleep with women, then your girlfriend should get to sleep with men. There is absolutely no reason you should restrict her sexuality if you get to fuck all the chicks you want.
As a last note, it can be difficult to find a partner to whom you both are attached. In my relationship, many women are much more interested in being with Mr. Evvie and they pretty much ignore me until they are forced to interact with me. Why? Because in my relationship I'm the 'slave', so they assume I should not matter and that I will like them. I am not a side dish. If you are going to be in a relationship with me, you will get to know me. I have seen women tell Mr. Evvie, "yeah I can't wait to play with Evvie! I've never been with a girl before
" and they've never even spoken a word to me. It amazes me that some people think it is acceptable to get to know one person in a poly relationship but not their partner.
In the end... I suppose I will never enter in to a strictly monogamous relationship. Poly has its ups and downs, but it's right for me.
(OP) I don't think I could make that work...
