I guess it depends on what you mean by extremely. It's not the norm, obviously. But there's no way to get real numbers, because people are so closeted.
Dan Savage suggests that successful open marriages are much more common than people think and filled a column with anecdotes to prove his point -- http://www.thestranger.com/seattle/S...e?oid=11412386
-- and this great article from The Advocate expands on the topic with some very interesting research and more anecdotes -- http://www.advocate.com/Print_Issue/...es/Monogamish/
My gf and her husband have been together for 10+ years and married for, I think, 4, and they have one of the strongest marriages I've seen (now, wasn't always).
So, I'd say false, but again it depends on how you slice it.