The Third Precept: I hereby accept the training rule of avoiding all Sexual Abuse.

One should cause No Pain to others or oneself in one's own search for pleasure...

Specifically:
One should not mate sexually with another's partner.
One should not mate sexually with engaged or married to another.
One should not mate sexually with minors < 16-18 years of age.
One should not mate sexually with those imprisoned, forced or under the law.
One should not mate sexually with those protected by family or teachers.

From the Buddhist Dictionary:
http://What-Buddha-Said.net/library/Buddhist.Dictionary/index_dict.n2.htm

kāmesu-micchācāra: lit. 'wrong or evil conduct with regard to sensual things';
Unlawful sexual intercourse refers to adultery, and to intercourse with minors or other persons
under guardianship. The abstaining from this unlawful act is one of the 5 moral training rules
(sikkhāpada) binding upon all Buddhists. Through any other sexual act one does not become
guilty of the above transgression, which is considered a great crime. The monk, however,
has to observe perfect chastity by living fully celibate - even without any masturbation.
In many Suttas (e.g. A.X., 176) we find the following explanation:
He avoids unlawful sexual intercourse, abstains from it. He has no intercourse with girls who
are still under the protection of father or mother, brother, sister or relatives, nor with married women,
nor female convicts, nor, lastly, with betrothed girls.