If a married man or woman propositions sexual misconduct to a person, or a man or woman propositions a person they know is married, is there a prescribed penalty according to the law, i.e. would a a court of law apply any punishment, in practice?
In parashat matot the beginning it says if a woman thought that her vow was still in effect, but it had been already nullified by the husband and there is no prohibition, and she "violates", she still needs atonement. This shows that even the attempt to sin needs atonement.
Rabbi Akiva commented on this: if we see even an attempt needs atonement, surely if one actually sinned that they need atonement. See Rashi there.