I mean, in my circle of people- we are pretty much all longtime-mostly happily married people (if they are unhappy, they aren't airing their dirty laundry, publicly, etc). Can't say I've really experienced it "close" to me, which obviously is a really good thing. I have never done it, etc.
That said, you look at the world, and it honestly if its over 50% that DID cheat, it would not shock me.
High divorce rates, lots of cheating and breakups- and honestly- these are the people that ARE getting caught and put out to dry. How many either don't get caught at all and "get away with it" (honestly the main reason why it DOES happen because people feel they can keep it under wraps), or do get caught, wind up being forgiven, but its all kept on the "down low" (and most rest of the public is none the wiser). So, I can see it not being too promising, overall.