It's a holdover from the 19th century, Hollywood plots, and cheap novels. In reality it's an accepted part of life in most of the world. Any successful man is expected to have a side piece, a GF, a mistress, or a concubine. With the possible exception of Jimmy Carter, all of our presidents have had mistresses or GFs, and they didn't even try to keep it secret. It isn't bad, just not socially acceptable for the working class.