Jewish Views on Premarital Sex
Judaism's attitude toward 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 mamzer (illegitimate). Nonetheless, marital sex is considered ideal, and premarital sex is traditionally not approved of.
The negative attitude toward 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 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.