This question could be much more easily answered if the Bible made a clear proclamation such as “living together before marriage or outside of marriage is living in sin.” Since the Bible makes no such definitive statement, many (including some who profess to be Christians) claim that living together outside of marriage is not living in sin. Perhaps the reason the Bible does not make a clear statement is that, in Bible times, the arrangement of unmarried people living as husband and wife was relatively rare, especially among the Jews and Christians. For the purposes of this article, when we refer to living together, we are referring to living together in the sense of living as husband and wife, including sexual relations, without being married. We are not referring to a man and woman living in the same house without sexual relations.
While the Bible does not make an explicit statement about living in together, that is not to say the Bible is completely silent on this issue. There are numerous Scriptures used to declare God’s prohibition of sexual immorality (Acts 15:20; 1 Corinthians 5:1; 6:13, 18; 10:8; 2 Corinthians 12:21; Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3; Jude 7). It’s important to note, that adultery and fornication are not exclusively relegated to sexual immorality. Jesus spoke of an adulterous generation.
“But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:” Matthew 12:39
This is a reference to adultery as those who worship the God of Israel AND other gods as did Israels worship of the idols of Egypt. Fornication is a reference to worshipping any god to the exclusion of the God of Israel. To be clear, God is not against sex, Solomon has many wives and a number of girlfriends.
“Of the nations concerning which the Lord said unto the children of Israel, Ye shall not go in to them, neither shall they come in unto you: for surely they will turn away your heart after their gods: Solomon clave unto these in love. And he had seven hundred wives, princesses, and three hundred concubines: and his wives turned away his heart.” 1 Kings 11:2–3
Consider the law of Moses, sex was treated as a covenant, its not how many wives or women a man has, its any deception or betrayal around the heart that is the issue. (to make one think they are the one and they are not, this is sexual immorality) (Deuteronomy 21:11-14) To be married and to hide that relationship is adultery, it is not the sex in and of itself, it is the lie. There are many biblical accounts of sex taking place in advance of marriage, in some instances as in Deuteronomy 21, sex does not always end in marriage.
“And this I speak for your own profit; not that I may cast a snare upon you, but for that which is comely, and that ye may attend upon the Lord without distraction. But if any man think that he behaveth himself uncomely toward his virgin, if she pass the flower of her age, and need so require, let him do what he will, he sinneth not: let them marry. Nevertheless he that standeth stedfast in his heart, having no necessity, but hath power over his own will, and hath so decreed in his heart that he will keep his virgin, doeth well.” 1 Corinthians 7:35-37
The Greek word translated “sexual immorality” or “fornication” is porneia, and it means literally “unlawful lust.” What would be considered lawful sexuality is the marriage of one man and one woman (Genesis 2:24; Matthew 19:5), anything outside of marriage, whether it is adultery, premarital sex, homosexuality, or anything else, would be considered unlawful. In this context, it is more about the intent of the heart, if it’s only about sex, what is the judgment of those who married and later divorced? Was the sex during the marriage immoral because you didn't say together? No, the intent of the heart is what matters, marriage is a necessary step to live out the credibility of the choice. There is no rationalization in living together if there is a commitment, to live together and not marry is to walk the line of indecision absent of commitment, which in and of itself makes the choice immoral from a biblical perspective.
“Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge.” Hebrews 13:4