While I personally would be open to having more than one person I feel emotionally primary to, my husband's boundary is that he wants to be my only total primary with whom I share a home, finances and child/children with (at least at this time he feels that way), and we really started exploring poly because he wanted to explore things in more of a swinging style, to enjoy pleasurable friendships of varying duration.
My boundary is that I am a very slow to warm up personality type, so swinging, at least the version of it that I saw on some of the boards (two dates and lets get it on sexually) felt uncomfortable to me. I like to hang out with people for a while, and know that we're compatible for more than sex, and that I want the person or persons in my life for long term with sex being only part of a lot of areas where we have fun, compatibility and emotional and intellectual connection. I could see doing something like swinging with someone(s) who are more friends than lovers, but they would have to be really good friends already. I do have room in my life for pretty much one main secondary relationship right now, but haven't met anyone yet.