I wouldn't say we were both in poly relationships before, I have had open, non-exclusive relationships where sex with others was not a problem, he has been more in relationships where sex with others was allowed usually in a BDSM way. I don't think either of us have ever had poly emotional attachments.
I also should make it clear that the relationship between me and BF is very new, late Feb, up until then we were supposedly mono. That was by default, he tells me now that he never realised that would be the case, I suppose I have a more conventional view. We agreed before BF and I got together that he could do non-sexual S&M with her, it is not unusual in our relationship for him to spank/beat other women, in clubs or whatever. He then facilitated BF and I getting together, we had been friends and jokingly flirtatious for about 10 years, but neither of us realised that the other actually fancied us, until he pointed it out. After she and I first slept together, he asked if I would mind him having sex with her in the aftermath of S&M play. I would not be comfortable with him having a romantic attachment, I feel that is something that has to be just for us, if only because I need that security from the father of my children. He doesn't feel that for me and other women (I wouldn't want to be with other men, and he wouldn't like it outside the odd D/s arrangement, so it's never been considered) he was very happy me and BF got together, he gets a lot out of knowing we are making each other happy.
When we first considered having a relationship, he asked me if I wanted marriage and kids, because that was his priority, to me that means mono unless stated, I don't think in hindsight it did to him, but at the same time he knew it was not OK for him to be with other women. When he first rang to ask me out he asked me to marry him, I told him not to be so daft, but from the very beginning it was on the cards. We had been close for a long time, talking through our relationship issues with each other and giving advice, we felt we knew a lot about each other in relationships, quite objectively. We both felt we had been fucking around (figuritively and literally) for too long, we wanted to settle and have a family.