*Definition of adultery: "The parties to this crime, according to Jewish law, were a married woman and a man who was not her husband."- Smiths Bible Dictionary.
*No prohibition of premarital sex under the Law: "Many people are surprised to learn that the Torah does not prohibit premarital sex. I challenge you to find any passage in the Jewish scriptures that forbids a man from having consensual sexual relations with any woman he could legally marry. It's just not there!" "http://www.jewishvirtuallibrary.org/jsource/Judaism/sex.html#Promiscuity (The Early Church regarded the OT as the only Bible and Rule of moral conduct.)
One reason that there was no law forbidding a man, married or not, from having sex with an unmarried woman was because sex itself was considered a valid means of taking a wife(or concubine) and polygyny was accepted under God's Law.
* Proof that sex alone was a valid means of taking a wife: "Mishnah Kiddushin 1:1 specifies that a woman is acquired (i.e., to be a wife) in three ways: through money, a contract, and sexual intercourse. Ordinarily, all three of these conditions are satisfied, although only one is necessary to effect a binding marriage." http://www.jewfaq.org/marriage.htm#Acquire Examples: Isaac took Rebekah by bridal price and sex, no contract. David took Abigail by sex/cohabitation. Gen.2:24 'leave and cleave'.
*Proof that sex was a valid means of taking a concubine (Really needs no proof, it's part of the definition of the term): Deut.21:10-14; Gen.30:1-4;
On the other hand, sex in itself didn't necessarily create a marriage; there were stipulations.
God gave His list of sex sins in Leviticus 18. It doesn't mention anything about sex with an unmarried woman. (See definition of fornication below) There is a case of a girl in Deut. being punished by death for not being a virgin on her wedding day, but knowledge of that culture tells us that she was betrothed, purchased as a virgin; her crime was a type of adultery. Another passage in Exodus demonstrates that sex with an unbetrothed virgin was not a sin; she was not punished. It was considered an act of potential marriage if the father approved.
There were no prohibitions of sex among singles living on their own. Men had sex with unmarried prostitutes without censure and that's the closest example I can think of, other than concubinage, to demonstrate that sex among non-marrieds was permitted under God's Law. Israel was a heavily patriarchal society in which females were almost entirely dependent upon males all of their lives, so it's difficult to make a comparison to today.
There was no requirement in the Bible for a male or female to be a virgin on their wedding day. However, men preferred to marry virgins, and punished adultery with death, because they had an obligation to provide for their own children, not someone else's children.
The tradition of chastity before marriage developed over time in response to various changes in cultures over the centuries. When the Church melded with Rome, it left behind biblical polygamy and adopted the Roman law of monogamy. Later, sex itself became viewed as sinful and celibacy/monasticism was regarded as holy and pure. The truth about sexuality and God's positive attitude towards it was forgotten and obscured. The Catholic influence led to the tradition of chastity before marriage by males and females; it didn't come from the Bible.
* Defining Fornication: Strong's #G4202 porneia, from verb #G4203 (porneo) meaning "to prostitute, to act the harlot" from #G4204 porné, harlot." Prostitution is the primary usage of the word in the N.T. (where KJV has "fornication", NIV has "sexual immorality") but the prostitution specifically being condemned was temple prostitution, see 1 Cor.10:8;6:13-18, "qedesha" in Hebrew. Occasionally the word "porneia" was used to refer to other forbidden sexual acts such as incest (1 Cor.5:1), adultery (Matt.5:32) and male homosexuality(Jude 7) The assumption that fornication means premarital sex (http://goldenrule.name/Fornication_English-EARLY.htm) has led to false interpretations of certain passages such as 1 Cor.7:2 and Hebrews 13:4