This is a really good question, sidhewolf. I only have a little bit of real-world experience, but here's my perspective:

Many, if not most, submissives seek the One. And want to be the only one for their One. I won't talk about the whys of it, there are plenty of threads here discussing monogamy vs. polyamory.

Finding someone who genuinely is not seeking that type of relationship is hard. And if they are, particularly if it's their first experience in poly, brings up all sorts of emotions and fears and concerns. As you know, open communication is critical.

My own experience? Milord and I are working our way into poly. It has it's ups and downs for sure. But we are separated by distance, and both want to feel free to see to our own needs and desires openly and honestly. In that spirit, He has recently begun a close relationship with another sub who lives closer to Him. She is a friend of mine, prior to the commencement of their closer relationship. I haven't yet found my comfort zone in the amount of information I need/want from the two of them about their relationship, so I am feeling my way in it. It does help, however, to know that He considers me His primary submissive and does not expect that to change. Just as He is my primary.

On my side of it, I have recently begun a relationship with a local Dom who has a poly family - a wife/slave and another slave. I am not and will not become his third slave, but am becoming his submissive. He is aware of my Dom, knows that I am committed to Him and am His first and foremost, and is happy to have me in his life as his submissive only. I have met his wife/slave - she cooked Thanksgiving dinner Friday evening for us and told him if he didn't get me over there she was going to be very unhappy! LOL That was strangely nice. I like her - although I must admit it is strange to sit at table and share a meal with a Dom who will have his cock down my throat in an hour...and his wife. It's just not part of my normal experience until now. I'm finding I like it - the openness. The acceptance.

I hope you find what you are seeking. It takes a special person to live an open, healthy poly lifestyle. I wish you the best of luck in your search.