It's a corporate governance problem because the relationship can be seen as tainting personnel decisions. But that's HR optics versus morality.
Note that I'm not defending Huang, but I think he's done for because of the spy thing more than because of this.
This is why you should only do one shitty thing at a time and not multiple.
It's inappropriate to use coercion.
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.
2. Other employees might feel that the employee having sex with the manager is getting better treatment at work.
Which, ironically, swaps the power imbalance.