I have to say, that we never really discussed any boundaries. I don't know why this is the case, but there never seemed to be a need for that. It was more the other way round: My husband was set as the one at whose pace we decided to move along (during the first weeks) and he was really fast. He never requested something like a set of rules and we naturally tend to check in with each other nowadays, if we think something feels awkward.
It would have felt really unnatural to me to set up restrictions for my relationships. It seems to be enough to bear in mind what would make me feel uncomfortable and ask the other parts about the specific point in question. We all seem to work that way and it works quite well. Maybe we will stumble at some point with this arrangement, but there are no rules or boundaries around at the time and we all feel comfortable with our situation at the moment.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.