The Torah (the Five Books of Moses) doesn't explicitly outlaw sex before marriage, but it doesn't approve of it either.
Judaism’s attitude towards premarital sex is intriguing. The Torah does not outlaw it — as it does many other types of sexual relationships — and the child of such a union is not considered a mamser (illegitimate). Nonetheless, marital sex is considered ideal, and premarital sex is traditionally not approved of.
Sex within versus outside marriage
The negative attitude towards premarital sex, to a large degree, reflects the overwhelmingly positive attitude toward sex within marriage. Marriage is referred to as kiddushin, which comes from the Hebrew word for “holy.” In Judaism, holy things are things that are set apart and made special and unique.
When sex is reserved for marriage, it too is considered holy.
Most Jewish authorities disapprove of premarital sex because it does not take place within the context of kiddushin.
What about long-term monogamous relationships?
What of a long-term committed sexual relationship in which two people — though not married — have designated each other as their exclusive partner?
This question has been raised by some liberal Jewish thinkers; however, both the Conservative and Reform movement (officially) reject the possibility of attributing kedushah (holiness) to such a relationship.
As mentioned, the Torah does not directly prohibit premarital sex.
Indeed, at times, rabbinic authorities and traditional sources have been lenient in this area.
In medieval Spain, Nahmanides permitted sex with an unmarried woman who was not involved with another man.
Nonetheless, for traditional Jews, premarital sex is not without halakhic (legal) complication. The Torah prohibits sex between a man and a woman who is menstruating (known as a niddah).
This prohibition is in place until the woman’s period is complete and she immerses in a mikveh or ritual bath.
This restriction applies to both married and unmarried couples, though it is considered inappropriate for a non-married woman (except for a soon-to-be bride) to immerse in a mikveh. Thus sex between an unmarried man and woman can violate a Torah decree.
Interestingly, the Torah does sanction one type of non-marital sexual relationship: concubinage. A concubine or pilegesh is a woman who, though involved exclusively with one man, does not receive the legal benefits of marriage.
In biblical times, concubines were kept in addition to a wife or wives. In recent centuries, Jewish authorities have, for the most part, dismissed the validity of concubinage.
An interesting exception is the 18th-century legal authority Jacob Emden, who suggested re-instituting the practice.
Calls for change
Many liberal authorities have pointed out the need to develop a new sexual ethic to address the reality of premarital sex.
Arthur Waskow, a leader in the Jewish Renewal movement, suggests altering our expectation of marriage to “make it easy for sexually active people from puberty on to enter and leave marriages.” The Conservative and Reform movements, while still stressing the ideal of marital sex, have acknowledged that Judaism’s position on human sexuality is not consonant with the trends of contemporary life, in which people often do not marry until their 30s or later.
Both denominations have suggested that premarital sexual relationships — where they exist — should be conducted according to the ethical principles that govern married sex: namely with the respect due to all humans as beings created in the image of God.
In addition, Conservative rabbi Elliot Dorff has stressed the importance of modesty, fidelity, and health and safety in non-marital sex.