I think that a dress low-cut enough to be considered unprofessional is a sexual issue by default. Professional standards and decorum exist, in part, to prevent a sexualized environment.

How is it judged that a man's pants are too tight to be professional, or a woman's dress is too short, and what are the reasons? The reasons are sexual, and I might be wrong in this, but I would guess the line exists where people of either gender would begin commenting.