To be responsible, the employer must have tolerated or condoned the harassment. At a minimum, the employer must know about the harassment. If so, the employer is responsible for the actions of the supervisor whether the employer knew about the harassment and regardless of how the employer responded. This makes sense because the supervisor used the power of the employer to hire, fire, etc. to carry out the harassment. Now, lets take a look at sexual harassment.
Sexual harassment is a special form of illegal workplace harassment.
All of the above information applies to all illegal workplace harassment including sexual harassment. There are some additional factors that are unique to sexual harassment. All this attention to sexual harassment has taken romance, chivalry and ordinary comradery between co-workers of opposite sexes out of the workplace. Sexual harassment is about sex. Sexual harassment is most often about power.