As I see it, F's true motivation isn't to corrupt D and then introduce her to the world of less normal sexual practices.
(normal as in the statically term rather than a statement of what is and isn't acceptable in any particular social-group.)
F loves D and wants to have a mature relationship with her, and as part of that he wants to introduce her to the sexual component of such a relationship. D has started feeling the same about F and is interested in learning more about being in a mature relationship with him.
Depending on the effect the particular story-line "Us The Player" has chosen to associate with, we may wish to foster a monogamous mature relationship between F & D, or we may wish for F to decent into the realms of corruption made available within the game, or we may want to do both!:evilsmile: