I think its because whether or not they admit it, there was a lot of ****ing around back then. Since there were no DNA tests, no cameras every where, and the wife was often trapped with a guy that cheated on her due to financial and societal constraints all they had was hate.
I had an older relative what was always bitching about divorce, after she died we went through her stuff because she hid money everywhere and we found our that her husband was divorced and she was "the other woman".
My grandfather on the other side of the family had three children out of wedlock. His one sister talked him into admitting one was his on the birth certificate and my grandmother hated that sister with a passion.