well it depends on what your relationship is based on.
My guy and I are poly, forcing the other one to be mono would never ever work, that would kill the realtionship as it has done for both of us so many times.
I love my guy with my whole heart, his is my number one and I am his, however I am mature enough and able to love more people with out forgetting my number one and so is he.
And yes when the time comes we will add on, we have a girl in mind, so then there will be a plus one.
He knows my secondary well, they have a laugh together and likes to drink beer together so friendship feelings all around.
I would never marry my secondary cause he and I doesnt have that kind of relationship.
I did marry my number one cause he lets me be me fully and loves me for who I am and doesnt want me to change.
Nice being with some one who isnt jealous or possessive for once and who lets me blossom and love HIM to the fullest, which makes him blossom too and loving to the fullest too.