From Nagarjuna's Treatise on the Great Perfection of Wisdom
(Dharmamitra Translation)

As for the precept against sexual misconduct, if the female is under the protection of the father, the mother, the elder or younger brother, the elder or younger sister, the husband as head of family, a son, the law of the world, or the law of a king, if one violates her, this constitutes sexual misconduct.

Sometimes there are those who, although they are not "under protection" in this sense, are nonetheless under the protection of the Dharma. How is it that one is under the protection of the Dharma? This refers to all women who have left the home life and to those who are householders but who have taken the "one day" precept. This is referred to as being under the protection of the Dharma. If one uses force, or if one uses money, or if one engages in deceptive seduction, or if one has a wife who has taken the precept, who is pregnant or who is nursing an infant, or if one engages in sexual activity involving an inappropriate orifice,-- if one transgresses in such ways, this constitutes sexual misconduct. All sorts of situations like these even extending to the giving of a flower garland to a courtesan as an indication of intent,-- if one transgresses in such ways, this constitutes sexual misconduct. If in all sorts of situations such as this one refrains from taking such actions, this qualifies as not engaging in sexual misconduct.