For me, it is all about what works - I am not fussy about configuration.
Right now I am in a developing relationship with a guy who has been in both mono and polyfi relationships. My long term relationships, until now, have been polyfi. At the moment our configuration is mono - but we barely have time for each other, so adding anyone else would be highly impractical. However, we are both open to the possibility to others - as you never know what life will bring - despite that neither of us is seeking.
So again, not hung up on any one relationship configuration, but very hung up on relationships that work.