Why does the bible never specifically condemn premarital sex? Most Jewish and Christian scholars define πορνεία (sexual immorality) to include premarital sex. However, where either the old or new testament mention specific sexual immoralities, it's never included.

In Leviticus 18 many specific acts are condemned, but not premarital sex. Likewise, Paul spoke specifically of incest in 1 Corinthians 5, adultery and prostitution in 1 Corinthians 6, and homosexuality in Romans 1. If premarital sex is indeed immoral, it is obviously the most prevalent form of immoral sexual acts by far, so then, why doesn't he repeatedly warn against it?
The reason premarital sex is not mentioned specifically is because engaging in sex was what consummated a marriage. In essence, the first time a couple engage in sex, they are married.  


  If a man find a damsel that is a virgin, which is not betrothed, and lay hold on her, and lie with her, and they be found; Then the man that lay with her shall give unto the damsel's father fifty shekels of silver, and she shall be his wife; because he hath humbled her, he may not put her away all his days.
  -- Deuteronomy 22:28-29 (KJV) 


This is the case with virgins who are not betrothed. Betrothal, on the other hand, is considered to be legal marriage in every respect, waiting only for its consummation by sexual union.


  Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost. Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily. But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost.


Joseph was compelled to "put away" Mary, because the evidence as he initially saw it suggested his wife had committed adultery.

Additional Comments

Paul, like every religious Jew who cared about righteousness, understood that Leviticus 18 was the yardstick against which all sexual activity was to be measured:

Leviticus 18

If you aren't in the green circle, says God, then the land will eventually spew you out "when ye defile it, as it spued out the nations that were before you. For all these abominations have the men of the land done, which were before you, and the land is defiled;"

God is a nation builder, and best practice in regard to sexual union is essential in order to achieve it.



Chapter 7 of 1st Corinthians is Paul's attempt to deal with various questions that had been put to him by the Church at Corinth, and the very first issue he addresses is immediately relevant to the concerns of the OP.


  It is good for a man not to touch a woman. Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
  -- 1 Corinthians 7:2


Paul is advocating celibacy, which he elaborates on later, but he understands that not everyone is like himself. So, here he suggests: if you desire to touch a woman/man, then find yourself a wife/husband in order to avoid fornication.

In Paul's theology -- if you're having sex and are not of the "one flesh" mindset, then you're a fornicator or an adulterer. He couldn't be any clearer.

Conclusion

In our modern culture, sex is much less about having children than it is about pursuing pleasure. This clearly wasn't true of the Jewish culture as we have it depicted in the Bible, where sex was mostly about building families.

The underpinning principle in regard to sex is found right back at the beginning of the biblical narrative:


  Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
  -- Genesis 2:24 (KJV)


This "one flesh" union can only happen ONE TIME.