I just lived this, and am still living it. For the full story (in progress) check out my tale in... General Discussions I think?
My conclusive decision is... there are no bad decisions. Indeed if I had just said "Okay, you're mono, I'm poly, let's not get into this, it sounds like trouble." then I would have spared us a lot of heartache, sleepless nights, endless writing, arguements and outside debate to name a few aspects. But then again, we would have also missed out on what I believe for both of us has been the closest relationship so far in each of our limited lifetimes, learning about ourselves and each other, being challenged to understand ourselves and each other. Adversity brings on great things and I would not go back and retract my decision at all.
I'm not saying your relationship is doomed; I'm not saying it will work. I'm saying if you take the difficult road and approach it with a sensible head, you will learn and grow no matter what the outcome. Avoid conflict and you learn nothing but how to avoid conflict.
Often he had been often bound with fetters and chains, and the chains had been rent asunder by him, and the fetters broken in pieces: neither could any man tame him...and he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.