I would think it rather rare.
With one affair, possibly. Serial, no, not unless there was a lot of questionable/borderline behavior leading up to it in years past. He may not even acknowledge that he was doing it or consider it cheating. Things like fantasy, flirting, "checking out" women to an unhealthy extent or checking out online dating profiles out of "curiosity", bad boundaries.
I just checked your profile for some history and knowing that you found online things, I would say there are a ton of gateways on the internet.
For my husband, he didnt actually physically cheat until around 10 years of marriage. But what lead up to that started with curiosity about adult dating websites, then fantasy, wanting to know what other women were like, building anger and wanting 'revenge', justifying why he should go ahead and just have some harmless chats, venting, ego stroking, to finding someone to meet... and so on. I think internally its usually a long path that gets them there, particularly when its multiple OW. And poor lifelong coping skills.
I also think its common to know only the tip of the iceberg.