Good question....only recently (6-8 months) did my wife of 27 years know I was poly or wanted to enter a poly lifestyle....you talk about a shock! For me, being poly with 2 women can only exist with love.....I don't believe in the "legal cheating" terminology. We are in a unique situation of all 3 being best friends and loving eachother very much. Having said that, my wife has surely been wrestling with the question you posed, "why can't she be enough"?
I thought intially it WAS about our individual and very different sex drives, I was wrong. It's more about completing or rounding out our "whole" lives. Each of the 3 of us has different wants and needs, economic, physical, emotional, etc. This just seems the best way for all of us to get those needs met, especially now as we move into middle age with kids off to college,etc. My wife is def not poly, but I think she's making rapid strides in adapting to it. Just yesterday, she hopped in the car with us and exclaimed, "life's just too short not to be happy"....this is a big change for her, and I'm sure she looks at it as though she somehow has been a failure. She hasn't been, she's a great wife. Some people do change however (that's me), it's nobody's fault.
My feeling is that in the future, with living expenses and natural resources getting tighter, we'll see many more family and communal living arrangements sprouting up everywhere. More nuclear families with grandparents, etc. Will everybody be poly? No, but some will and will live, fuller, richer lives for it. Hope this helps you a little.