There are a few things about your post I'm not quite clear on but I'll give it a go.
Yes I think I am what your asking about. A mono female in a poly relationship and there are definitely others who regularly contribute to this board. I don't quite understand what you mean by "crossing the line"? I started out as mono, I have played around with the idea of taking on other lovers (male and female) but my heart really isn't in it. My poly partner satisfies me and any other potential relationships that have come into my life have largely felt like shouldering a burden. Is this that what you mean? Getting to the place where you define yourself as mono? On the other hand I do like the idea that if a miraculously wonderful relationship did find its way into my life I would have the freedom to explore it. Does that mean I haven't quite crossed the line? I don't think so, it's just about being open to what the Universe decides to offer up.
The reasoning behind acceptance of differing needs Z feels enriched by other people in his life in an intimate way, whereas I feel drained. Well mostly drained. Let's say I haven't yet got enough out of another relationship to make it worthwhile pursuing. I have a very full life, there really isn't room for any more emotional entanglement. When I did decide to have a bit of an explore I was doing it because I thought it would ease the hurt I still carry around Z being poly. Maybe it was a process I had to go
through in order to cross the line you talk about into being comfortable as a mono in a poly relationship.
Z needs other women in his life, partly because he doesn't relate well to men on level beyond the superficial. I think this has something to do with being raised by a mother and older sister.
I am learning how to love myself.
Oh and I started a whole blog up for mono's in poly relationships. While there are only a few female monos on this forum there are lot of us out there. Most hang out on the mono/poly board at yahoo groups.