Boss-subordinate relationships are a lot like conflicts of interest. You can’t really rule out something nefarious without the ability read minds because coercion and self-dealing can happen at an unstated or even subconscious level.

1. An employee might feel obligated to have sex with their manager knowing that it might affect their careers.

2. Other employees might feel that the employee having sex with the manager is getting better treatment at work.

There cannot reasonably be consent when a subordinate is involved. The power dynamic prevents the relationship from being on an equal footing.

propriety is a higher bar than what you're referring here. it IS inappropriate to date in that power dynamic. but it is legal. and it can even be ethical with mutual consent and management of consequences. dont confuse them.

All adults are equal, but some are more equal than others.

> Adults can consent to sex with other adults.

This is questionable in the context of one of the adults holding a position of power. Also, even when assuming the adults are peers, there's another saying 'Don't shit where you eat' which is rooted in a lot of stories of workplace relationships going wrong in very creative ways.