In the case of Yaw, he was not expelled for adultery. There was allegations from one of his conquest that she felt compelled to provide additional service to maintain her employment. Yaw failed to turn up to answer questions and did a runner. In fact, WP sort of treated it as a private matter in the first instance and only acted after the fresh allegations came about.
The PAP had through out its history including in recent times had cabinet ministers and MPs screwing around, having kids out of wedlock and if no serious abuse has taken place, the parties are told to sort out their own matters and not to make it a public spectacle. The Civil Service gets hundreds of complaints each year usually from aggrieved spouses and the vast majority is left to the parties to sort it out themselves. Sometimes the superiors step in to counsel if the office work is affected.
The point is that one should not be led by the state controlled media by the nose. Nobody proceeds to have an expensive by-elections because surprise surprise!! a man and woman had wild sex in the privacy of a room.
The state controlled media will never ask the PM if infidelity or adultery is a dismissible conduct. And you know why?