OK so now I get it. If you go and have a look at Kat Tails blog, or the blog of the mono wife in the blogs and lifestyle section, you will see that while we accept we find actually being "comfortable" a lot more difficult. Likewise the mono/poly site but you probably wouldn't want to bother registering.
I would say that most mono women fall in love with a poly before they realise how the implications of that will feel. It's one thing to know it intellectually but a whole different kettle of fish to experience the emotional roller coaster ride. That was my situation. The other typical scenario is that an existing partner falls in love with someone else while still loving the original party.
In both these cases we accept and try to make ourselves comfortable because of the quality of our relationship and the level of love we feel for our poly mate. I think you have to have a certain level of emotional intelligence, open-minded and interest in personal growth.
If we broke up I can't see myself knowingly going into another poly relationship.