Why would it lead to more than them getting fired? What would be the crime? If the idea is that they've coerced/raped a prisoner, presumably that prisoner is making that allegation, and it's the rape allegation that gets investigated/tried, as with any other serious allegation of a crime. If it's consensual, that sounds like they're just not trustworthy to do their job and should be let go.

No, the prisoner did not make an allegation.

But this is the point I am making, if I have a relationship with someone at work, particularly when one person is subordinate or position of authority, then we'd simply be fired... but a prison officer will face a criminal prosecution of Misconduct in Public Office, and possibly face prison time themselves. Seems somewhat unfair for a low paying job.