I'm late to this thread, so I'm just going to throw out a few random things from my personal situation that are different from what others have said. Not to argue, but to show that there is more than one way to poly.
When Hubby and I opened our marriage, we started with more of a "swinging" model. Sex okay, friendship okay, romantic feelings not allowed. Our aim at that point was to keep our marriage the priority, in large part because of my kids. We *agreed* that if one of us developed romantic feelings for another partner, we would cut contact with that person and would come clean to each other. Although I've seen from many posts on this forum that such an agreement is impossible to maintain and isn't fair to each other or to other partners, it was what Hubby and I felt was necessary. When we had other partners, we made them aware right off the bat that we had this agreement.
We also gave each other veto power, but with the condition that we could only veto family members (such as cousins), friends and coworkers, and people we worried might be a threat to the marriage or to one of us. Family members were right out. I don't have any coworkers; Hubby works for his father and doesn't want his parents aware of the arrangement, so his coworkers were off limits (and of no interest to me anyway).
As far as the "threat" thing... one example is a woman that's part of the group we met through, who is known for--and proud of--breaking up at least three marriages. I vetoed her; Hubby wasn't interested in her anyway. I also ended up vetoing the one other woman Hubby did have sex with, because I found out she was telling others in our group that he was a stalker. (She was having family issues and he texted her a few times over about 3 weeks to make sure she was okay. Apparently to her, that was "stalking.") And Hubby vetoed one of my friends with benefits when we found out the FWB was using me as a back-up plan, to the point of constantly breaking plans with me because someone more interesting came along, and of telling another woman he was involved with "It's okay if you can't see me, I'll just call KC because I know she'll come over."
When I realized I was in love with another of my FWBs, I went to Hubby and confessed, and told him I would cut ties with the guy. That was also when I was finally able to articulate to Hubby that I was polyamorous, and *he* was actually the one who first used the word. (Thanks, Robert Heinlein!) At that point, Hubby had decided he wasn't interested in having even a purely sexual connection with anyone else. But he told me that I didn't have to cut ties with the other guy.
One of the most important things about boundaries and agreements in poly, to my thinking, is to make them negotiable. When Hubby and I opened the marriage, we actually wrote a list of our agreements and boundaries, and one of them was "These are not set in stone; everything's negotiable."
So when I came out as poly, the "no romantic feelings" agreement was abandoned. Hubby understood that I might fall in love with someone else, and was willing to accept that as long as our marriage remained intact. My FWB thing (which at that point didn't even have benefits; the guy had been in our area on business for several months, but two months earlier had gone home, 800 miles away from us) became a relationship. That relationship ended in October, after just over a year; it couldn't survive *not* being long distance when my boyfriend came back out here on business again.
With my first boyfriend, we did have a hierarchical model, mostly because my boyfriend was long distance. It didn't make sense to me to give him equal place in my life as Hubby when I lived with Hubby and almost never saw my boyfriend. However, with S2, that's changed. Although Hubby does have a certain priority by virtue of the fact that we live together, share finances, and have been raising my kids together, I've made it clear to him and to S2 that I do consider the two of them equal as far as their places in my life. Neither is more important, they're just important in different ways, and I consider them co-primaries. And S2 and I have certain agreements and boundaries as well.
With both S2 and Hubby, I have agreements that I will be told if they decide to look for another partner, whether for a relationship or just for sex. If they find someone else without planning it, I'll be told. If they have sex with another partner, I'll be told, and condoms are mandatory with any other partner until that person's been tested. (They only have to tell me about the *first* time they have sex with someone else; after that it's kind of assumed there'll be sex with that person.) And they will not cancel plans with me to see someone else. Of course, these agreements also apply to me, but at this point I have no intention of bringing any other partners into my life.
As for cohabitating... to be honest, I'd be happy if I didn't live with *either* of them. Sometimes I just want to not trip over another person when I'm trying to get things done. I live with Hubby because we're legally married and, as I said, my kids live with us. S2 has said that for a number of reasons, he's finding that he prefers living alone and just having me spend a couple of weekends a month and a couple of evenings a week with him. He says he wouldn't mind having me over more often, but that right now, he doesn't intend to ever live with a partner again.
