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.
I have a case in the courts right now where a woman supervisor hired a young boy as a stock clerk because she wanted to have sex with him. There is another case on appeal in California where the female chief financial officer required her male subordinate to have sex with her as a condition of his employment. The company must have 15 or more employees for each working day of 20 or more calendar weeks in the year of, or preceding, the unlawful act. Under California law, all companies are liable. A company with just one employee is subject to the California anti-discrimination law, called the Fair Employment and Housing Act.