Hi Zelda, welcome to the Forum! I have only been here for a few months myself and equally new to poly - but have found a lot of solid advice and good info here. I am sure that you will also hear from some of the veterans as well.
You are in a tough, but apparently not too uncommon a situation, in which one partner desires to be poly (or to make the relationship poly) and the other is opposed to the idea. It is quite the conundrum if you both love each other - especially if to the point that breaking up may be just as painful as foregoing your poly inclinations might be - yet neither partner is able to forego their hopes for the relationship (be it poly or mono). There is probably not one best answer - and the answers you receive will probably depend upon the natural bias of the one you ask, be in mono or poly. Ultimately, it would have to come down to some careful soul searching to determine which is more important - both now and in the future, a decision best not made in haste it would seem. Obviously, the answer will vary by individual.
The dilemma becomes even more pointed if you are cohabiting, married, or have kids - so I would say that it is good that you have recognized your leaning for poly before things have gotten more serious - so you can make the best decision that you can now before it goes any further - and becomes even messier and more painful down the road.
Mockingjay mentioned the possibility of further discussion with your bf - and that might be worthwhile. I was very satisfied with our happy mono marriage (and we were both in agreement that there was nothing wrong with our marriage) - when my wife asked me to open our marriage so that she could explore her resurgent feelings for her an old college bf. (There is a link to the full story in my signature below). I was not very happy about this suggestion (to say the least) - but did eventually agree to work toward acceptance of the idea (and we are well into it now - my wife will be going on her third overnight with her bf next week - and I have achieved a great deal of acceptance - and even compersion for her happiness - even if it still would not have been my first choice.)
I posted this earlier for someone else but will repeat it in the event that you might find any of these concepts helpful in discussing the issue further with your bf:
Here's some of what my wife, Becky, did right in discussing the possibility of opening up our marriage to poly:
1. Made sure that I understood that it was not about me - no matter how much it might seem that way. There was nothing wrong with me and I had not failed in any way. This was about her feelings and not a reflection on me.
2. Made sure I understood that it wasn't because she didn't love me as much as she ever had - but she believed it was possible to love more than one man at the same time without diminishing the love for either (the classic poly argument is to consider that a parent can love multiple children at the same time and still love any others than come along as well).
3. Made certain to emphasize that her relationship with Ben would be "in addition to" - and not a replacement for - our marital relationship. And that our relationship would still be "primary" (after all we were married with a child in a home - with a mortgage, etc) - and that we could negotiate what exactly that would look like. I found the phrase "in addition to" to be particularly helpful in helping me come to terms with her desire to have a second relationship.
4. She was very patient - and did not try to push it along too quickly. We could take it one step at a time, and she agreed not to have sex until I felt I could accept the idea. And although she did want to include sex in their relationship eventually as a natural progression, for her it was much more about her feelings and her emotional involvement.
5. She was willing to talk about it as much as I needed to - no matter how long it took or how often - and to answer any questions that I might have - without any apparent frustration. She understood the need for me to process the situation. And she was very patient and understanding of my frustration and emotional turmoil at her request - very validating while still upholding her belief.
6. She went out of her way to reassure me of the depth of our relationship during this time - avoiding arguments, being especially loving and affectionate as well very open and honest about here thoughts and desires.
7. She agreed that being completely honest and transparent was absolutely essential - to whatever degree that I felt I needed that to be ok.
8. Made sure that I understood that I could also have another partner if and when I was ready.
And, I already understood from life experience that it is very possible to love more than one person at a time (acting on that is the issue) - and that one person cannot supply all of another's needs. These points may need to be discussed as well.
There is probably more that I could write about with a little thought - but Becky really did do a great job in asking me to open our marriage to poly. Hopefully some of this may be helpful to you as you discuss things with your partner.
Best wishes for a successful resolution - whatever direction that goes!
Al