All that has really changed is that it used to be "out of sight, out of mind". Mistresses and affairs have never been uncommon, but they were relatively discreet. People just care less about appearance now, and I would guess that is in part because people are more anonymous now in many ways.
I don't think so. There are a large number of people who take their cues of write and wrong from what is socially unacceptable. Not just in sex, but in honesty, courage, etc. Once a practice starts to spread they jump on the bandwagon.
We are social beings, and we have social tendencies. One of them is to be influenced by the society we live in.
There were always some who were loose, but once it became socially acceptable there were more.
Shalom.