Me and my husband have talked about this alot as well.
"Each wife/woman/partner was in charge of a specific duty. Today a wife has to be a good mother, a good spouse, good at her job, social, take care of herself and be pretty, have food ready for the family and so on and so forth and all that with a smile on her face."
Not to mention that in a monogamous relationship you have to be their lover, mother, nurse, friend, team mate, therapist, coach. In short when your only two you do lean VERY hard on the other person. Me and my husband after many years of marriage have agreed that our love has indeed grown for each other and is stronger but still cannot be the end all be all fulfillment of everything we need. Its not possible.
Now I have a husband and a Dom. I love them both. However because of the commitment if hubby said no dom I would. Earlier in my life I could have never imagined getting to this point. But Im glad I did. The 3 of us are much happier!!
That being said. Being honest and transparent. Doing alot of planning and negotiating is very important. Communicating and sticking to our agreements is VERY important. I hope the situation lasts a long time. This is my first time. Im ok if it doesn't though because right now were all happy and getting our needs met.