it's funny enough, after watching Industry on HBO, one of the characters fukks a coworker before his wedding and tells her "he needed to get the poison out".
That's really what it comes down to for most of us in these situations...you never planned on cheating but work stress mixed with life/home stress manifests itself in that way. Add in the fact that overtime, you get more familiar with coworkers and shyt happens.
If that redhead is who he cheated with, not saying it's right (but I understand why just like Clinton & Lewinksy). You're around someone enough, you grow fond of them and truthfully no one expects you to be cheating with them cause they themselves wouldn't cheat with them. No the finest bytch at the office, everyone has their eyes on her, so you make a move, everyone will see. A regular chick, well you can talk with her, get lunch with her, and folks will assume you're just being nice.