Answer:
   
   The Torah typically frowns on premarital sex. Some extreme statements
   have even been made, for example, Reish Lakish has stated that even
   one who sins with his eyues may become an adulterer (Lev. Rabba 23);
   however, this never became accepted. However, this attitude led to
   many of the traditional separations between man and women, such as men
   not walking behind women, men and women being separated on festive
   occasions and in public parts, and even separate days for visiting
   cemetaries.
   
   However, this question is not focusing on the traditional separation,
   but the attitude towards premarital sex. The literature makes it clear
   that virginity for the female was prized. Intercourse with an
   unmarried girl generally fell under the concept of Zenut, which was
   prohibited. If an act of intercourse was intended as an mode of lawful
   bethrothal, it was considered to be a lawful betrothal (Mishna Kid.
   1.1). Although the act was prohibited, children born of such liaisons
   were free of any blemish, and there was no question of their legality
   (Kid. 4.1,2; Yev. 100b). Nachmanides was lenient about such illicit
   unions, and was willing to overlook them (Isaac b. Sheshet, quoting
   Nahmanides, 6, 398; also 425 and 395).
   
   What about sexual relationships between those who were engaged and
   might live together for some time. This has been prohibited by
   tradition (Shulchan Aruch, Even Ha-ezer 55.1). In early times, such
   intercourse was reported as unobjectionable in Judea, but not in the
   Galilee (Ket. 7b, Ket. 12a). As for the children, some felt they
   should be declared Mamzerim (Yev. 69b; Kid. 75a), but this view was
   never adopted.
   
   Note that the discouraging of sexual relations outside of marriage is
   a property of all Jewish movements. The Reform Responsa on the subject
   explicitly states:
   
     On the question of informal heterosexual relations outside marriage
     between two consenting single adult individuals, we can then come
     to the following conclusions. Such relationships were prohibited
     and discouraged by authorities throughout the ages. Little was done
     when such relationships took place between two engaged persons,
     except in puritanical periods. Other sexual relationships between
     single adults were prohibited, and every effort was made to enforce
     such prohibitions. These prohibitions were equally strong upon the
     man and the woman. In times of lower moral standards, authorities
     were occasionally permissive or simply looked the other way.
     Generally, the effort to enforce high moral standards succeeded,
     and the responsa call attention to the failures. In our own period
     of loose standards, it would be appropriate to do everything within
     our power to encourage higher standards for both men and women. We
     should do whatever we can to discourage casual sexual relations.