The "Reasonable Person" Standard
If an unwelcome behavior of a sexual nature causes someone to take offense, it will be judged based on whether a "reasonable person" would find it offensive.
This standard of a reasonable person has arisen from court attempts to interpret what behaviors should reasonably be considered sexual harassment. Since not everyone interprets behaviors in the same way, the courts find that, in order to be illegal, the conduct must be severe or pervasive and offensive to a reasonable person in similar circumstances.
Under this standard, one-time unwelcome behavior will seldom qualify as sexual harassment unless it is sufficiently severe as judged by a reasonable person.