If one employee kept telling another employee sexual jokes that the second employee found offensive, it would be sexual harassment. If two employees dated and engaged in consensual sex, this would not be sexual harassment. If one of the two then wanted to terminate the relationship, and the other used the unequal relative terms and conditions of employment of the work place to further the relationship, this would be sexual harassment. Jokes, pictures, touching, leering, unwanted requests for a date have all been found by courts to be sexual harassment. Sexual harassment can be between people of the same sex.
Sexual harassment can be a woman harassing a man.
Anyone who is offended by a sexually harassing environment may theoretically sue. However, that employee's offense must be reasonable. An extremely sensitive person might not be able to maintain a claim, because her feelings of having been offended were not reasonable. The reasonableness is evaluated by a standard that is the same as a person in the victim's circumstances. For example, what a reasonable woman might think is a hostile environment is not necessarily the same as what a man might think is a hostile environment.