Sex outside the marriage relationship is sinful.
We have previously established that:
- GOD designed life's proper order.
- GOD created marriage.
- GOD created sex.
- Marriage is a lifelong commitment.
- Marriage is between a man and a woman only.
It isn't politically correct or popular these days, but the Bible asserts that woman was created for the purpose of helping man. In other words, women were created to bless men.
That does not mean that GOD cares less about women than men. Nor does it mean that men should be selfish and treat their wife like a slave. Quite the opposite. The Bible is clear about how men should love, honor, respect, and treat their wives.
Nonetheless, because ancient people understood that woman was created for man, it was common for women and daughters to be treated differently than men and sons. In many ways, culturally, daughters were seen as a sort of property or "items of value" which could help their husband, in everything from homemaking, to cooking, to child-bearing—bringing forth many sons, of course, to caring for their husband's needs, and much more. This is why dowry was a tradition in many cultures.
Because of this reality, in many ways, marriage is GOD's design for ensuring women were properly taken care of. A man was to leave his father and mother and be joined to his wife and, in so doing, the two formed a new household and family. In this way, daughters would be cared for in a world where physical strength was often the key to success and survival.
Therefore, GOD instructed men to make a lifelong commitment to the woman they chose as their wife. GOD was looking out for "the weaker vessel."
The protections of this commitment—along with other reasons such as avoiding the spread of sexually transmitted diseases, optimal household performance, and ensuring a healthy and loving environment to raise children—is a major reason why GOD didn't simply allow any man to have sex with any woman.
Ultimately, whatever His reasons, GOD says that sex is only allowed within the marriage relationship. For example, the Law of Moses contained severe penalties for a woman who married and deceived her husband into thinking she was a virgin:
13 “If any man takes a wife and dwells with her, and hates her, 14 and lays reproachful words upon her, and brings a bad name on her, and says, ‘I took this woman, and when I came to her, I found she was not a virgin,’ 15 then the father and mother of the young woman shall take and bring out the evidence of the young woman's virginity to the elders at the gate. 16 So the young woman's father shall say to the elders, ‘I gave my daughter to this man as wife, and he hates her. 17 Now he is laying reproachful words upon her, saying, “I found your daughter was not a virgin,” and yet these are the evidences of my daughter's virginity.’ And they shall spread the cloth before the elders of the city. 18 Then the elders of that city shall take that man and discipline him; 19 and they shall fine him one hundred shekels of silver and give them to the father of the young woman, because he brought a bad name on a virgin of Israel; and she shall be his wife; he cannot divorce her all his days. 20 But if his word is true, and evidences of virginity are not found for the young woman, 21 then they shall bring out the young woman to the doors of her father's house, and the men of her city shall stone her to death with stones, and she shall die, because she did a foolish thing among the children of Israel, to commit fornication in her father's house. So you shall put away the evil from among you.
(Deuteronomy 22:13-21 OSB)
Similar prohibitions were included in the Law for adultery, prostitution, incest, and homosexuality.
Later, under the new covenant, sex apart from marriage was equally strictly forbidden. Here is one such example:
4 Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge.
(Hebrews 13:4 NKJV)
GOD considers it a sin to have sexual relationships outside of marriage.