It is not illegal for a boss to ask an employee for a date or for them to actually date & have sexual relations, or for co-workers to do the same.  The problem comes if the boss uses this relationship to control the employee in some manner, like a quid pro quo in getting raises & promotions being tied to the relationship in some manner.  Even if both parties are happy with the arrangement, it can be wrong to deny raises & promotions to others who aren’t having sex with the boss who are as qualified or more qualified.

It’s also a problem if the employee uses the relationship to falsely claim they were harassed & that there was no legitimate consent.  This is essentially extortion, but it’s hard to prove, and the boss and company wind up on the short end of the stick virtually every time unless a conspiracy or other exculpatory evidence comes to light.   This is what happens many times in sexual harassment claims, and the companies & bosses usually pay them off to avoid the negative press & costs of litigation or the risk of getting a judgment against them.  For every Harvey Weinstein, there may be just as many or more that didn’t do anything legally or morally wrong who get sued or even arrested for it.

Just because an employee didn’t want the boss to ask for a date even 1 time, it’s not sexual harassment.  The harassment could only start after that if the boss continues asking or the boss does something retaliatory due to the rebuffing.  The boss could be fired for violating a “no dating” company rule, but that’s not legally actionable.  Well, you could sue someone for anything with no evidence, but it doesn’t mean you’ll get a settlement or win the case; you could also be countersued for legal expenses by the person or entity you sue, and you could be charged with a crime yourself if you perjured yourself creating false charges or a prosecutor believes they could prove extortion.