If you have to ask that question you obviously don’t manage staff at any level.
Even if the whole world knows it’s consensual, the subordinate can still sue stating they were taken advantage of etc and in many circumstances win. I know of a case where they LI e together for years, have several children and she still sued and won. Crazy
Plus the boss and a subordinate will ALWAYS make people feel like they are second fiddle and create tensions for assignments, promotions etc. and typically leads to mother lawsuits and claims.