Hope you don't mind me weighing in, but I think this is a pretty common point of difference and has a lot of variation across age and region.
I'm not sure the reality has a lot of difference, but how open about it people are definitely does. For me dating, it was widely understood that people had "someone" they would call regularly despite being single, and, in all likelihood, that if a date went badly, they were probably going to call that person after. "Single" didn't mean "not having sex with anyone", it meant "not having restrictions on who you are having sex with". That was typical even many years before Tinder and all that.
For other people that comes as a shock, but it's been my experience that even with people who believe it isn't the norm, they still do the same thing, just with more discretion, and think they're the only ones misbehaving.