What is the Biblical basis for prohibiting sex outside marriage?

The crime of adultery is clearly prohibited in the Ten Commandments, but it is true that there is no specific commandment in the Old Testament prohibiting fornication. Prostitution is unlawful however (Deut. 23:18), as is seducing a virgin (Ex. 22:16). This covers most expected cases in OT society - a woman was presumed to be a virgin when she married - but it does leave a loophole in cases where a woman is no longer a virgin, is not a prostitute and is not married.

The New Testament, on the other hand, does prohibit fornication. In Acts 15, James the "Lord's brother" states:

19 It is my judgment that we do not trouble those who are turning to God from among the Gentiles, 20 but that we write to them that they abstain from things contaminated by idols and from fornication and from what is strangled and from blood. 21 For Moses from ancient generations has in every city those who preach him, since he is read in the synagogues every Sabbath.”

Nearly all translations use either "fornication" or "sexual immorality" for πόρνεια in vs. 20. A few translators substitute "unlawful marriage," so those who wish to justify fornication may appeal to this interpretation.

Conclusion: while fornication in itself was not a punishable offense in OT Law, no circumstance was envisioned in which it would be sanctioned. It is clearly prohibited in the New Testament. Sex within marriage was the first blessing God gave to man (Be fruitful and multiply). Sex outside of marriage contradicts God's greatest gift to us.

My friend is a Progressive Christian who says that the bible doesn't condemn or even mention sex outside of marriage in the bible.

For your friend to say that, she has not really read God's word and should probably do so before making conclusions that could get her pregnant. Does her progressiveness also allow her the quick way out, which is to take the life that she irresponsibly conceived out-of-wedlock? God calls that exactly the same as what Cain did to Abel, according to the book Harbinger II.

Should she suddenly marry the man she had a one-night stand with? I ask these tough questions because this happened to me, so I know that of which I speak.

Tell her it is because God knows what's good for society, since he made Adam and Eve and the covenant of Marriage, and sex can get her pregnant, and that child should remain alive, be brought to term, and live in a healthy family with married parents that are as committed to each other and God as they are to their child. And she would find all of that throughout the Bible if she would prayerfully read it.

Golden Rule: Matthew 7:12 (NIV) So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets.

Another way to look at this is the Golden Rule, which is the standard for all morality. Pretend she has sex and gets pregnant, but the guy is (fill in the blank). Now switch places with the child in the womb. Do you want your mom to abort you? Do you want your mom to give you a difficult life because she raises you herself, not as part of a team? Do you want your mom to marry the guy anyway, and have one mess (unhappy marriage) or the other (divorce) because the foundation of the marriage isn't solid? There's no good way to look at it for that child just accidentally conceived and probably unwanted. Do you want to be unwanted?

Now perform the Golden Rule again, but with the happy marriage scenario. You're a child in the womb, wanted, with parents who work together as a team, and stay together, laying down their lives for each other and submitting to each other in Love as instructed in the Word. If you are both following Jesus, it is easy for you to sacrifice for the other when needed. And who would not want to have a marriage like that? Or be a child of that marriage?

Therefore... First produce the family that your daughter would love to be part of. Then you can have the sex that might give you a daughter. Anything else leads to the fatherlessness spoken of in the last chapter of Malachi, the same fatherlessness which is bringing on the natural disasters we are seeing more of, again from Malachi.

Malachi 4

1 [a]“Surely the day is coming; it will burn like a furnace. All the arrogant and every evildoer will be stubble, and the day that is coming will set them on fire,” says the Lord Almighty. “Not a root or a branch will be left to them. 2 But for you who revere my name, the sun of righteousness will rise with healing in its rays. And you will go out and frolic like well-fed calves. 3 Then you will trample on the wicked; they will be ashes under the soles of your feet on the day when I act,” says the Lord Almighty.

4 “Remember the law of my servant Moses, the decrees and laws I gave him at Horeb for all Israel.

5 “See, I will send the prophet Elijah to you before that great and dreadful day of the Lord comes. 6 He will turn the hearts of the parents to their children, and the hearts of the children to their parents; or else I will come and strike the land with total destruction.”

If you read the book Harbinger II, you will see that the guidelines and laws God gave to Israel are for all of us, with some of the corrections and clarifications brought by Jesus when he otherwise ratified the Law and the Prophets by saying

Matthew 5:17-19, 27-28 17 “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. 19 Therefore anyone who sets aside one of the least of these commands and teaches others accordingly will be called least in the kingdom of heaven, but whoever practices and teaches these commands will be called great in the kingdom of heaven.

27 “You have heard that it was said, ‘You shall not commit adultery.’[e] 28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.

If your friend wants to read one set of things that the Bible says about sex outside of marriage, they can read all of Proverbs 5.

A note about the Harbinger II. While it evokes some strong emotions in some, there is no real voice of God (prophecy) that does not have that effect, no matter where you read in the Bible.

The Harbinger II itself is really a call to prevent or reduce any more "shakings" like 9/11, the Financial Market Crash, or Covid-19, by doing the following:

2 Chronicles 7:13-14 NIV 13 “When I shut up the heavens so that there is no rain, or command locusts to devour the land or send a plague among my people, 14 if my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then I will hear from heaven, and I will forgive their sin and will heal their land.

Don't we want our land healed of drug addiction, lantern flies, wildfires, inflation, STD's, sex outside of marriage, school shootings, monkey pox, et. cetera? Our plagues are multiplying like crazy. Yet I do not see anybody organizing the body of Christ to do this as a group.

A long time ago, I was part of a mass Christian prayer meeting in Washington DC, to fast and pray for our nation. And Harbinger II includes details about what God did as a result of that specific meeting that I was a part of -- President Ronald Reagan, as he was being sworn into office, placed his left hand on his mother's bible, on 2 Chronicles 7:13-14, the same verses as above -- his mom had written next to those verses something like "A most wonderful scripture for the healing of the nations." This detail was in Harbinger II -- I never knew this until I read the book.

God's enemy will use any tactics at his disposal to get you not to read this book. So disobey God's enemy, instead of God, and get this most excellent book, and read it. I am starting my third time through, this time with a notebook!

And the sin of abortion, the mass murder of the innocents, is directly related to this question, and is strongly in the book.

Just think... Because New York started all of this, worldwide, One Billion murders worldwide have taken place. And God, who has created every single soul of that One Billion, does not take that lightly!

The phrase "sex outside of marriage" is a strange one from a biblical perspective.

And Isaac brought her into his mother Sarah's tent, and took Rebekah, and she became his wife; and he loved her: and Isaac was comforted after his mother's death. - Genesis 24:67

One must read a long way into the Scripture before anything approaching a wedding ceremony is detailed. For most of (biblical) human history the copulatory act and the marriage were not separate events. And I'm not convinced God has changed his mind about it.

Know ye not that your bodies are the members of Christ? shall I then take the members of Christ, and make them the members of an harlot? God forbid. What? know ye not that he which is joined to an harlot is one body? for two, saith he, shall be one flesh. - 1 Corinthians 6:15-16

Failure to recognize this fundamental nature of marriage has led to a world of adultery.

It's one of the 10 commandments: Thou shalt not commit adultery (see Exodus 20:14)

The 18th chapter of Leviticus is devoted to condemning all manner of other sexual sins, lest any seek for loopholes that weren't quite covered by the previous commandments. Deuteronomy 22:21 clearly condemns sexual relations before marriage.

Even entertaining sexual fantasies is condemned by Jesus:

But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. (Matthew 5:28)

From the very beginning God's standard is abundantly clear:

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)

This sacred union between a man and a woman--becoming one--comes after marriage, not before.

Post-script

I will disagree with the view that in the eyes of God (or His people) the act of sexual intercourse is what made a couple "married".

These thoughts may be very unpopular (if you're looking for it, the down vote button is above and to the left), but they are Biblical. Biblically, marriage is supposed to precede sexual intercourse, not result from it. Five (of many) examples cited below:

• Genesis 2:22-25. Before Eve is brought to Adam she is referred to as a woman; after God brings Eve to Adam she is referred to as his woman (what we would today translate as "wife"). God brought them together as husband & wife (what we would today call "performed a marriage") well before any intimate relations occurred between them. It was the giving/committing of one person to the other, not sex, that resulted in marriage.
• Genesis 29:21-23. Joseph is clear is his discussion with Laban that Laban's daughter must be "given to him" before there are any sexual relations. A feast is held, and then Laban "gives" his daughter (sneakily giving Leah not Rachel) to Joseph. Consummation of the marriage comes afterward. It was the giving/committing of one person to the other, not sex, that resulted in marriage.
• Genesis 38:14-25. In this polygamous society Judah could marry more than one woman, and Tamar is eligible for marriage. Yet their sexual act is condemned as unrighteous, and does not result in marriage. Neither gave themselves to the other -- these sexual relations between two people not married to each other was what today might be called a "one night stand" and it was considered a serious sin.
• Deut. 22 is painfully explicit with respect to sexual sin and the punishments associated therewith. While Israel did use what we would today call "shotgun weddings" in certain cases, the couple did not become married as a result of having sex, they were married later, after the man paid the girl's father the bride price. Verse 21 is unmistakably clear that the damsel who is unchaste before marriage is guilty of great sin.
• The Bible repeatedly condemns fornication (e.g. Matthew 15:19, which condemns both adultery & fornication)

For a much deeper dive on the topic, these videos on my channel may be of interest:

• On the historical context of marriage, divorce, and adultery: Divorce & Remarriage: unpacking the Bible’s teachings in their historical context
• Chastity, from a practical & theological perspective: The Law of Chastity - What the World Doesn't Want You to Know