here is a link to the NF's -
I think that it is good to feel something - sometimes relationships can conjure up an emotional reaction that may not have been brought to the surface otherwise. This can give you an opportunity to identify with it and work through it instead of around it.
I have definitley noticed that I can feel deeply for more than one person at a time, although not much has happened physically with regards to those people. Just this fact - that I can have these emotions gives my SO those wormy feelings, and the same thing happens to me if he feels that way about someone.
The thing is, that i can only speak for myself in expressing that the support for internal growth from him makes me cherish the existing relationship. Sure - it can be confusing at times having these feelings for more than one at a time, and I do wonder if I am doing something "right", "wrong", or whatever.
My history of relationships was very much a one to the other very quickly and with emotional overlap almost every time... serial monogamist to the core! I think that I felt guilty about feeling for someone else, always wanted to explore it, but didn't know that there was another option other than just breaking it off with my current partner. Hence the questioning of my actions now - it is hard to face myself like this.
In sum, work through this stuff. It is a great journey even if just for yourself. It is a gift that you can carry with you forever to build confidence within yourself, and learn to be happy for others.
Even if a relationship style this extreme is not what you want forever, there are great things to gain! Especially since your SO seems nice from your description. I would think that your ultimate priority is to be sure you are healthy first.