An extramarital affair in the FBI is in the headlines lately. One was an agent and the other a lawyer working on the Russian influence investigation. If they are having a “secret” affair, obviously, the most dramatic reason for that would be that they are married to other people. That may make them seem bad, but we’ve had plenty of protagonists who are engaged in affairs throughout the history of fiction and drama.
However, if they are keeping it secret because they are working together and don’t want to be separated from an investigation, that is a different kind of bad. It makes them seem unprofessional.
Now, considering the reality of how cop shows and movies present law enforcement, it honestly doesn’t need to be realistic at all. There are very few actually realistic portrayals of actual procedures in law enforcement even in shows that are based on real cases. So, you don’t really need to worry about plausibility in the context of actual policies.
Rather, plausibility is what you create in the dramatic and emotional reality of the situations. If you set up the risks, rewards and conflicts of the relationship consistent with the narrative, then people will buy it. No one ever cares if the story is realistic if it isn’t dramatic.