I have had many conversations with many professing believers who have argued to me that God does not disapprove of “shacking up” (i.e. a couple living together like they are married without actually being married). Their arguments have ranged from, “we're engaged so what's the big deal”, to “we're getting a feel of what it would be like to determine if we're ready for marriage”, to “the Bible doesn't say we can't live together”, and so on. Many professing believers do not see a problem or any carnality or compromise with “shacking up” (living together) with whom they're dating. Many professing believers may not even know how the Bible addresses this or if it even does or care if it does.
Let me say it first, the Bible does address this topic but not directly like a “Q&A”. You can find the Bible's view on this in it's pronouncement on marriage.
“18 The Lord God said, “It is not good for the man to be alone. I will make a helper suitable for him.”....21 So the Lord God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs and then closed up the place with flesh. 22 Then the Lord God made a woman from the rib he had taken out of the man, and he brought her to the man. 23 The man said, “This is now bone of my bones and flesh of my flesh; she shall be called ‘woman,’ for she was taken out of man.” 24 That is why a man leaves his father and mother and is united to his wife, and they become one flesh. 25 Adam and his wife were both naked, and they felt no shame.”” (Genesis 2:18, 21-25, NIV)
To physically move in and live together with your boyfriend or girlfriend is outside of God's design, and that is what makes it an error. God designed here in Genesis 2 that a man will leave his parents and be joined to “his wife” as “one flesh”. Living with your boyfriend or girlfriend purports becoming “one flesh”. How so? Because both people have left their separateness to live and be together as a couple (as “one”). A couple living together is a component reserved only for God's design in marriage. It is not meant to be done outside of this specific God-ordained relationship; for to do so is superseding God's design with one's personal view and preferences.
I have no need to address the potential and promising problems with couples living together before marriage to build an apologetic against it, because the greatest argument against it is that God ordains from the very beginning of humanity's existence that leaving, cleaving, and living together as “one” is for marriage only. To do otherwise, whether knowingly or unknowingly, is a clear defiance of God's design. One's innocent ignorance or willful ignoring does not dismiss their violation of God's design. It will always be a violation—i.e. sin (falling short of God's standard).
So, to all the professing Christians that may read this, you have no biblical “omitted” basis for your defiance of God's design for living together as a couple; neither can you use the “cultural context” excuse to explain away the truth principles in this passage because there was no culture defined. This was humanity's inception. Thus, this principle is tied to all generations of mankind in every culture. There is no escape. The only reason you have to stand on is the real one, selfishness and pride—you want to do what you want to do and you don't want to be told that you can't. After this biblical response, to attempt to argue that it's not a violation to shack up (i.e. to say God does approve) further shows your selfishness and pride in your effort to justify your sin.
If you are a professing Christian shacking up with your boyfriend or girlfriend, God does not approve and you are in fact sinning against your Savior and Master. As always, there is grace—of which you've probably been receiving from God in the midst of you sinning against Him and have likely mistaken it for His approval. God has forgiven you already in Christ, and you can experience that forgiveness in this area if you repent (1Jn. 1:9).
My final word to you, thank God for His grace and forgiveness, choose to obey Jesus and either get married, move out, or suffer the consequences of violating God's design (and whatever may come from that).