I have a friend who works in a very male-dominated work environment and sometimes they forget she is there. Boys talk and last week she told me that out of 20 of them, all married, 18 have mistresses and they are quite open about it between themselves. She was warned about the men before she started the job - in her boss's words: "they'll love you...be careful".
I'm (almost) divorced and been on dating sites for a year now and have been propositioned by many many men who openly admit they are married, and these are just the ones that admit it. Usually you can tell because they have no picture on their public profile.
I've been shocked by the amount of cheating that goes on, I must be so naive, I had no idea it was this bad, and its made me really pessimistic about finding a future partner. On the sites I just get messaged by guys who just want sex, guys who are married, guys who are odd, guys who are mentally ill. Internet dating just seems be be a massive hook-up site now and no-one seems to have a shred of moral fibre, its a nightmare.