I think that there are certain social or working groups where it's really common and therefore a lot easier to do, be forgiven and be accepted socially.
Which isn't to say that it never happens elsewhere, only that people are far less likely to have opportunities to cheat and it's less likely to come out.
One of my best friends fell in with a couple of women who openly cheated on their long term partners whilst at university. (One of whom is still with the man she cheated on, and he cheated on her multiple times also)
Our "home" group gave her extensive support about one awful relationship, and it came out via those friends at her hen do that she'd cheated on her ex with her fiance (who also had a fiance) for MONTHS before she told us they started seeing each other - and she cheated on the fiance too at a home friend's wedding.
She looked mortified, and we had to keep under wraps how shocked we were whilst the woman who spilled the beans crowed about "we've all been there, haven't we girls".
But I think it's hugely varies how prevalent it is. There will be people who say it's common and that others are deluding themselves, but I'd say equally that there is also an element of self-delusion going on there. Because being sexually faithful is seen as superior, it's comforting to consider "failure" to be common, and to hold superior knowledge.
Not unlike plenty of other circumstances where people prefer to believe worse of everyone instead of making a better effort themselves.