In addition to sharing children, op highlighted this being a ONS, not a fling or affair as a reason for continuing her marriage.
But realistically she was a work colleague, an acquaintance, one who was in the scene enough for op to actually be aware of her a d be aware she was interested in her h etc., not an anonymous stranger in a bar, for example.
And how exactly does one ramp up (and down from) to a ONS with a work colleague without anything inappropriate, flirtatious, sexual, illicit etc. It seems like it's likely to have been in the direction of a fling even if they really only had sex once.
It's noteworthy that not only has the ops h waited out 3 years (?) because, you could imagine, the length of time in the past, could be used to ameliorate the impact (and it's a not uncommon strategy) but also because it meant ow was no longer working around him or even in the country.
No hassle for him around "you see her at work, how do I know you won't start something, I can't stand you being around her, you need to change job etc etc."
It's quite calculating, self preserving, manipulative etc. and I honestly wonder about his disclosure being totally honest/self sacrificing.