Today's moral "climate" allows any and everything regarding sex, male with male, female with female, bestiality, and in fact, it is the same as described at Leviticus 18 concerning what the nations did sexually, with Moses detailing the immorality that was so commonplace in the land of Canaan, and of which Moses told the nation of Israel: "Do not make yourselves unclean by any of these things, for it is by all these things that the nations that I am driving out from before you have made themselves unclean. Therefore, the land is unclean, and I will bring punishment on it for its error, and the land will vomit its inhabitants out."(Lev 18:24, 25; Note: God's personal name is Jehovah, Ps 83:18; Isa 26:4)
Then Jehovah told Moses, saying: "Speak to the entire assembly of the Israelites and tell them, ‘You should be holy, because I, Jehovah your God, am holy."(Lev 19:2) When God created Adam and Eve some 6,000 years ago, he established the marital arrangement, that of one man as a husband with one woman as his wife, she being "a helper for him, as a complement of him."(Gen 2:18)
And in making this arrangement for both companionship and procreation, he also established that sexual relations was to be only between husband and wife, and not prior to their formal commitment of a holy union in the eyes of Jehovah God, for the woman is derived from the man as a husband's helper and complement when they are married.(Gen 2:21, 22; 3:20)
In order to be holy in Jehovah's eyes, a person had to maintain moral cleanness, and when looking at the original standard of marriage established in the garden of Eden, it is not difficult to see what he is pleased with and what he hates.(see Rom 1:26, 27, 32)
But since the rebellion in the garden of Eden, mankind has deviated sexually, such as seven generations down from Adam, a man named Lamech taking two wives for himself, instead of having only one as originally designed by Jehovah God.(Gen 4:19)
Sexual relations outside of the marriage defiles it, for it is no different than having sexual relations with a prostitute, to whom a person is not married. The apostle Paul wrote to the Corinthians Christians, who once felt that sex with any and everyone was okay: "Do you not know that anyone who is joined to a prostitute is one body (or "one flesh") with her ? For “the two,” says he, “will be one flesh.”(1 Cor 6:16; Note: to "Corinthianize" meant to practice sexual immorality. This sensuality was a product of Corinthian worship, particularly of the goddess Aphrodite (counterpart of the Roman Venus, the Phoenician and Canaanite Astarte, and the Babylonian Ishtar)
So, in having sexual relations before the formal arrangement of marriage corrupts it, being the same as having sex with a prostitute, for only when a man and woman are married are they "one flesh" and not before.(Gen 2:24) At Hebrews 13, Paul again expresses this: "Let marriage be honorable among all, and let the marriage bed be without defilement (or abstaining from pre-marital sex), for God will judge sexually immoral people and adulterers."(Heb 13:4)
Marriage is a binding covenant or contract before Jehovah God, and any altering of it, as in pre-marital sex, pollutes both partners. At 1 Thessalonians 4, Paul writes: "Finally, brothers, just as you received instruction from us on how you should walk in order to please God, just as you are in fact walking, we request you and appeal to you by the Lord Jesus to keep doing it more fully.......For this is the will of God, that you should be holy and abstain from sexual immorality (such as pre-marital sex). Each one of you should know how to control his own body in holiness and honor, not with greedy, uncontrolled sexual passion like the nations have that do not know God. No one should go beyond proper limits and take advantage of his brother (or a man or woman who they say will marry) in this matter, because Jehovah exacts punishment for all these things, just as we told you previously and also strongly warned you. For God has called us, not for uncleanness, but for holiness. So, then, the man who disregards this is disregarding, not man, but God, who gives you his holy spirit."(1 Thess 4:3-8)
Thus, Revelation 21 states: To anyone thirsting I will give from the spring of the water of life free. Anyone conquering (or being "holy", morally and spiritually clean) will inherit these things (of living on the paradise earth as a "meek" one, Matt 5:5), and I will be his God and he will be my son. But as for the cowards and those without faith and those who are disgusting in their filth and murderers and the sexually immoral and those practicing spiritism and idolaters and all the liars, their portion will be in the (symbolic) lake that burns with fire and sulfur. This means the second death (or everlasting destruction).”(Rev 21:6-8)
Hence, just as a person cannot legally drive until they have a driver's license, so likewise, a man or woman cannot have sexual relations until they are legally married.