Reaching a compromise with a mono boyfriend
I met my current boyfriend while I was in a monogamous relationship with another man. He and I instantly became best friends and were virtually inseparable. We kept our friendship entirely platonic for well over a year, but the emotional attachment we felt for each other was frankly undeniable. When I became single, it did not take long for the two of us to start sleeping together (really, just sleeping - there was no sex or anything else for a long time). We trust each other about everything, because we are each others' best friends. If there is such a thing as soulmates, he is undoubtedly mine.
Throughout all of this, I have been struggling over my feelings for a mutual friend of ours. Although I recognize that a relationship with this friend would not be as whole and complete as the one I have with my current boyfriend, it still hurts to have feelings of love and affection on which I cannot act. This friend feels the same way, although he admits that he respects just how special my current relationship is. But it still hurts to be around him.
A short time ago, I heard about polyamory and began reading into it. Bam! Everything clicked. The fact that I have feelings for this friend does not in any way subtract from the love I have for my boyfriend. I know this, and even my boyfriend knows this, as he indicated when I confessed everything. He told me that he had been aware of my feelings all along, and recognized that it didn't mean that I don't love him as much as possible. However, when I tried to discuss the idea of opening our relationship into a polyamorous one with this mutual friend, my boyfriend became hesitant. He wants me to be happy, but admits that it makes him very sad to think about me becoming emotionally intimate with anyone else. For him, part of what makes our relationship so special is that we're the only one for the other. I understand that, and I agree to a certain extent: he is the only one for whom I will ever feel this attached.
I guess what I'm looking for is advice. Breaking up because he is not "wired" the way I am is not really an option I am willing to consider right now. I love him very deeply, and if this hurts him too much, I will settle into monogamy and leave it alone. But what I would really like is to reach some sort of compromise. How do you propose a polyamorous relationship with a monogamous "soulmate?"
P.S. Sorry for the length and purple-y prose. Apparently, it is nearly impossible to write about love and emphasize its depth and meaning without resorting to words like 'soulmate' and 'attached.'