There are some clever variations here on the idea of a chastity 'keeper' who has sex with other men while her 'victim' has none. And having chaste guy wash the boyfriend's car is a nice touch. He could also polish his shoes.
Once the girl has the key, she can let her imagination take flight. And once she realizes the power she has ... I suppose the idea of fucking other guys while she makes her victim go without is very seductive.