What is "sexual misconduct" (kamesu micchacara)? Here are two definitions in the Buddha's own words.
"One conducts oneself wrongly in matters of sex; one has intercourse with those under the protection of father, mother, brother, sister, relatives or clan, or of their religious community; or with those promised to someone else, protected by law, and even with those betrothed with a garland" (Book of Tens, Anguttara Nikaya, X, 206).
"Abandoning sexual misconduct, one abstains from sexual misconduct; he does not have intercourse with women who are protected by their mother, father, mother and father, brother, sister, or relatives, who have a husband, who are protected by law, or with those already engaged" (See Bhikkhu Bodhi translation, In the Buddha's Words, p. 159, based on MN41; Saleyyaka Sutra; I 286-90).
Sexual misconduct or "wrong sensual indulgence" is karmically harmful behavior. On account of karma it will result in suffering and unsatisfactoriness now and/or in the future.
Any excessive or addictive sensual (kamesu) indulgence can constitute "misconduct" (miccachara). Strictly speaking, however, the term "sensual misconduct" is defined only in sexual terms, as the Buddha made clear by his definition.
Right Speech is abstaining from speaking unseasonably (at the wrong moment), falsely, harshly, idly, or maliciously. After all, the honest truth -- or the situation as one sees it and claims it to be -- may be more harmful when spoken in any of these other ways than even a fib or silence.
The problem with thinking that kamesu micchacara only refers to sex is that one soon becomes accustomed to the shorthand and neglects to consider that kamesu really refers to all five senses.
It is mistaken and off-putting to confuse the Puritanical teachings of other religions with the ethical universals taught by the Buddha.
Consenting individuals not under the protection of others are free to engage in and enjoy sex without being admonished. What is important is that in so doing they are neither being harmed nor causing harm to others.
The state (or monarch) may decree someone as "off limits," such as the common early English translation of a "female convict." This meant someone who was not free under a mandate, for example, a court edict or royal decree.
It would be clearly be harmful to engage in sexual intercourse with someone under duress. That would be harmful to all three parties -- oneself, another, and both (which means the community). In the same way, to a lesser degree, harm is being done when one has sex with someone promised [by parents or guardians] to another, betrothed, formally engaged, or married.
In brief, one avoids doing harm by abstaining, not from sex, but from "misconduct." To over extend or distort the meaning of misconduct leads to hypocrisy and even guilt for those trying to live as Buddhists.
Celibacy is not imposed on independent adults. It is sometimes voluntarily adopted as a form of training and self-discipline to bring craving under control.
- For more on this subject, see Buddhism and Sex by the eminent British scholar Maurice Walshe (Buddhist Publication Society, Wheel No. 225).