While premarital sex is considered sinful by the rabbinic community, the rabbis also recognize the human desire for sexual interaction. Sexual intercourse between a man and a woman is to be reserved for their union upon marriage when they become one with God. Several centuries ago, rabbis deemed the age of eighteen the proper age for marriage. This custom was put into effect to mitigate the desire for premarital sex. The only limits placed on sexual activities in the Torah are prohibitions against adultery and incest.
In biblical times, a man was not prohibited from having sexual relations with a woman, as long as it led to marriage. The Bible never explicitly states a woman and man may not have sexual intercourse prior to marriage; therefore, no sanction was imposed for premarital sex, but it was considered a violation of custom.
In Jewish law, the principles of modesty, zenut, forbid women from being promiscuous. A man must concentrate on his daily studies and praying, rather than being concerned by the sexuality of a woman. Thus, the rabbinic scholars of the Talmud prohibited prostitution and premarital sex (with no intention of marriage).
Sources: Eisenberg, Ronald L. The JPS Guide to Jewish Traditions. PA: Jewish Publication Society, 2004; Kolatch, Alfred J. The Jewish Book of Why/The Second Jewish Book of Why. NY: Jonathan David Publishers, 1989; Wigoder, Geoffrey , Ed. The New Standard Jewish Encyclopedia. NY: Facts on File, 1992.