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As a general principle, love and sexual desire must be directed towards building a sacred relationship. It is forbidden to direct them anywhere else. The most common type of prohibited sexual behavior is adultery, defined by Jewish law as sex between a man and a married woman. This grave offense betrays the marriage covenant and destroys the wonderful, holy power of love.
Incest is another type of prohibited sexual behavior. It includes sex between a man and his close relatives: his mother, his aunt, his sister, his daughter, his brother’s wife, or his father’s wife. Additionally, once a man is married, his wife’s close relatives are forbidden as well – her mother, sister, and daughter. Incest also includes sex between a woman and her close relatives or her husband’s close relatives.
Male homosexual sex is prohibited. Bestiality is prohibited. These too are extremely grave transgressions.
Non-marital sex is prohibited. It does not matter whether the woman is single, widowed, or divorced. If she is also a nidda, the prohibition is more severe. Nevertheless, as long as there is neither incest nor betrayal through adultery, a child born as a result of such relationships is not a mamzer. (See the next section.)
Masturbation is also a sexual transgression. True, it is a less severe transgression than the others; many authorities consider it to be only rabbinically forbidden. It is still prohibited though. This is because the sexual drive is meant to be directed exclusively to the holy relationship of a husband and wife; it should not be wasted. For the same reason, it is prohibited for a woman to masturbate in a non-marital context. This prohibition includes lesbian sex (which Jewish law views as mutual masturbation).
