The reason it took so long was because there was an “investigation“. An affair between two consenting adults wouldn’t be ”investigated” by the AD. They say “wtf is wrong with you”, maybe tell them to look for a new job, but most likely cover it up as it’s not really news.
The answer is that this resignation doesn’t necessarily have to do with a completely legal and non-policy violating, albeit immoral, act. But rather something that is violating some kind of rule that required an investigation and a determination.