I find that an interesting idea. The white, young, middle class being the "generic" polyamorist. The young part makes sense ... as my generation is much more accustomed to the idea of different lifestyles than previous ones ... though I would think that pretty much anybody who came of age during or after the 60's has had enough time to acclimate to a society that accepts non-traditional lifestyles.
The middle class also makes sense, since honestly, at the lower end of the economic spectrum I don't think they have the time and energy to invest into discovering the possibilities and educating themselves about what an adult relationship could be. And I say this as a professional who teaches adults in the lower income levels how to use a computer mouse on some days. They're super nice, but their time seems to be consumed with getting by, and polyamory takes time and energy for studying and contemplation and soul searching that they simply don't have.
The white part I don't get. But that might just be because I'm Asian and my husband is Hispanic. We've met/interacted with gays/straights/polys/etc of all races, possibly because we're West Coasters and ethnic ourselves we tend to mix with middle class people of all ethnic backgrounds.