It is not distaste that is the argument against professors dating students. It is blackmail as well as academic integrity.

Did she pass just because she did a sexual favor? Is she being lenient on the argumentation of this paper just because she is dating the student?

These are legitimate arguments and concerns, and while I think the rules set up to stop these problems are stepping well over their bounds, let's not try and act like there wasn't a problem to being with.