Except that the reasons behind those rules against sleeping with employees is the many, many cases of abuse that have actually happened. Bosses who make it quite clear that the price for a favorable performance review is a sexual favor of some kind. There's even a nickname for this kind of thing in Hollywood — "the casting couch" — and when there's a nickname for it, it's common enough behavior that everybody knows it happens. Not that every single person is doing it, but it's an open secret that it happens a lot in Hollywood.

And it's wrong. At its worst it's rape-by-coercion, trying to hide behind a thin layer of plausible deniability. It's not a circular argument at all: the behavior is bad for objective reasons, and that's why it's forbidden. In more favorable cases (where there's no coercion involved) it's still trading something of monetary value for sexual favors, which makes it a form of "prostitution lite". Again, there are objective, non-circular reasons for it to be forbidden.