I had a coworker who at the time was just the girlfriend of her now husband. This was over 10 years ago. We became really cool, and ended up going to Vegas and I smashed regularly because she loves her husband but isn't in love with him. At any rate, they eventually got married and we would still smash from time to time - but we're also really good friends. She only married him because he's a good looking guy, good job and her parents really felt like she should. But their chemistry is not there (ours is muuuuch stronger) especially this long into the marriage so she steps out periodically. She even mentioned divorce once, but he basically just ignored her and hasn't brought it up again, lol. They have a kid which further complicates things. They also moved into the burbs of LI (she's from the city) and is bored to tears out there.
Other than that, I had a co-worker who was married and she got traaashed at a holiday party and was throwing herself on numerous co-workers including myself. However, she was so drunk that I wasn't going to try to take advantage of that. They got divorced a few months later.
I've also known about numerous co-workers having affairs over the years.