Story - wise, it can lead to a breakdown of the social order in the fictional setting. It might cause scandal and ostracism for the parties involved. For example, in a period - drama type of story, the family's reputation would be severely damaged.
Well, in story - telling, there could be a lot of emotional turmoil. The servant might feel used and betrayed, and the person who engaged in the act could face moral dilemmas. There could be a chain of events that unfold, like the servant leaving the service, spreading the word and causing a stir in the community. It might also be a catalyst for other characters to re - evaluate their own values and relationships within the story.