It does indeed exist. I'm in a relationship with my long term boyfriend who is monogamous (meaning he is happy to be romantically involved with only me), and he believes he'll feel this way for the foreseeable future.
However, he does not impose that kind of model upon me. He accepts that that is the way he experiences love, but that its not necessarily how I experience love. Which is exactly the case. I'm in a loving relationship with both him, and another man at the moment.
If he decides he doesn't want to be monogamous anymore, then I would support that decision and be there for him in that case as well, but for now, indeed monogamy does exist in my poly relationship.