My husband and I were just discussing this last night. He has said that he adamantly does not want to benefit from my boyfriend, in the sense that if C is visiting, he can't help me with projects or gardening or whatever around our house. My husband is also unwilling to befriend C. He also hates the idea that any increase in my sex drive might be due to C, even though he knows there is a correlation. (I have not felt so sexually free and excited and fulfilled since extramarital intimacy, even without full blown sex, became a possibility.)
I do feel like this whole poly journey has had a positive effect on my marriage, because it has forced us to move from a rather unemotional autopilot to closely examining what we mean to each other, how we want to love and be loved, and what we hope to gain from our marriage. My husband feels these things could have been discovered without the discomfort of bringing polyamory into it. I doubt it, since it's such an inherent part of my nature.
My husband feels he has lost a part of me, which just breaks my heart. I've pressed him for specifics about what he has lost, but he has a hard time explaining. I kind of think the part he has lost is me being monoamorous, but that part wasn't real. I feel like I've gained a part of me, and am more complete than ever.
In essence, I think the benefit to the mono partner might be a spouse who feels happier and more complete, has a better sex drive, and is more conscious and in tune to her/his spouse's needs, and a marriage that is more consciously, thoughtfully loving through forced introspection and communication. I just don't think a mono partner is necessarily going to see or acknowledge those benefits unless he/she wants to. I'm hoping it will come in time.
Married to a monogamous man 15 yrs, mother of 2, dating C 3 yrs, and in a romantic friendship with L more than 20 yrs