For myself-I don't feel I NEED a closed relationship. But both my men do.
In general they satisfy me and so I don't much think on it at all.
It isn't "closed" to just us three, it's closed to me having both of them and they each have the abillity to have a girlfriend. If hubby wanted to share, so be it-but right now my heart is... not ready for a woman. Yes I'm bi-but emotionally not there.
When I was younger and my NOW bf was still a virgin, I was dating a woman. The three of us talked quite seriously about moving together (she had to move for school). She and I talked about her desire to have a baby (I already had a daughter and they were helping me raise her) and her thought that she wanted him to father it. She wanted to verify if it was ok with me since officially I was her girlfriend. Of course it was ok with me.
We had intention of having a closed triad. I love(d) them both and they both loved each other and me. C wasn't sexually involved at that point-but there was no doubt in our minds he WOULD be.
It never came to be for completely other reasons-but for us why it would have been a closed triad was because they both are mono by preference and while they could handle the idea of three of us-they couldn't handle the idea of anyone else having someone they were in love with.
"Love As Thou Wilt"