The "Reasonable Person" Standard
Definition: The "reasonable woman/person" standard is used by courts to assess whether particular conduct is reasonable by asking whether a reasonable woman or man would find it offensive.
Questionable behavior of a sexual nature will be judged, not by its offensiveness to the complaining person, but by whether it would be offensive to a "reasonable person."
Most of us will stop behaviors if we know they offend others. However, there is no rule that says unwelcome behavior must be repeated to qualify as sexual harassment. Instead, the "reasonable person standard" will apply.
If you have any doubt about a behavior being unwelcome, don't do it. Avoid the risk of offending a co-worker and the consequences of sexual harassment.