I remember reading something about the Russian studies a looooong time ago and that they did find that the number of dual marriages was surprisingly common among the different types of relationships, but still not a majority. With 14 different type relations there's a 7% chance of any given relationship. I thought they found that maybe 30-40% of married couples were duality which bucks the trend big time, but that's still not a majority. I haven't paid much attention to this lately though, so maybe I'm wrong and there's some newer studies out.
Not scientific, but is there any member here who's in a married Duality?