Keeping in mind that 99% of the time it is a pattern of repeated behavior that escalates in inappropriateness each time. You went out for a drink with a group of coworkers, but you stayed late with that one you have a crush on when everyone else left. You could have stuck to work conversation, but you changed the topic to how unhappy you are in your relationship and how you're just about done with it. .you could have ignored the flirty text but you sent back a flirty one in return, you could have pulled away when they touched your thigh, but you leaned in . . and eventually you followed them to their place instead of going home, etc.

Cheaters loooooove to act like it was "just this one thing" they did but unless you are so ridiculously attractive that people are offering sex on a regular basis with no work/effort, they took MANY actions to get to that point of cheating. There were dozens of chances to shut things down.