Thisisstillfolk your post got me thinking!
I owned a pub for a while and I would say that 90% of the men including the very married men who came in there tried it on with me at some point. Despite the fact that I know their wives. I think that most men, after a few drinks, might be tempted, even if it is not being offered, they will just chance their arm. At the time I had just split up with my husband and was new to the area. I suspect they had taken bets.
I never went near any of them by the way, but it was an interesting experience, repeatedly being propositioned by very married men. Guys who go to church with their family each Sunday etc
In the past, men were out of the home more than women, so had more opportunities. Now most women are equal in that regard and more financially independent, meaning we do not have to put up with crappy relationships for the sake of financial survival.
I guess the next question is...does it matter if your partner has a ONS?
When I was young I used to think it would be the end of the world, it was my biggest fear relationship wise. But then my husband cheated on me (went back to his ex, but then returned to me - but lied about being with his ex). To be honest it was not a happy experience but also not the end of the world. We stayed married for 20 years.