The issue comes to explaining motive.
From the student perspective: You reject an overture from your teacher, then the teacher grades you in a way that you deem to be unfair. You may then protest that you feel that the teacher graded you unfairly because you rejected his or her overtures. Not exactly the type of thing that any school would want to have to deal with. And frankly I wouldn't want to have to deal with it as a student either.
From the teacher perspective: A student rejects your overtures; later, you decide that their work isn't up to par and give them low marks or even a fail. The student protests as I mention above.
Unless you have some sort of bullshit grading system that allows teachers to assign arbitrary grades with no objective basis, it should be relatively straightforward to determine if the work was graded correctly or not. If the work is graded fairly, there is no issue. If it was graded unfairly, then proceed to punishing the teacher as necessary.
Edit: And as a teacher, if you decide to hit on a student and they shoot you down, well, you had better be extra careful when grading their work to make sure they don't have any basis for a complaint in the future.