I almost wonder sometimes if it's like when you buy a red car, suddenly you see red cars everywhere. We are so hyperfocused, it DOES seem to be everywhere. Having said that...it DOES seem to be everywhere for me, too! But again, I just turned 39, and most of the people I interact with are in the 35-55 age range, so maybe it's just part of my station at the moment.
To me, calling something an MLC affair has a lot to do with the other things going on for the MLCer. They do tend to be more involved than just sexual affairs, and last longer, but at the same time they have a juvenile quality. I'm not the expert though, I just know my H has been very clear to me that his reasoning for it had to do with feeling very alone and he felt OW had similar problems. Last time he spoke of it he continued to downplay it all, though he presents to her that it is a big true love story. Even when I said to him that I thought it was a big true love story, he laughed and said, "Oh, no...NO!".
I believe if it was legitimate, he'd legitimize it.