Sex outside of marriage is called fornication in the Bible.
For those who have believed the gospel and been baptised into Christ, fornication must be avoided because it will exclude from the kingdom of God.
Galatians 5:19 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, 21 envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
1 Corinthians 6:18 Flee fornication. Every sin that a man does is outside the body; but he that commits fornication sins against his own body.
Ephesians 5:3 But fornication and all uncleanness or covetousness, let it not even be named among you, as is fitting for saints;
Living with a boyfriend and avoiding sex is not wrong in itself, but it is inadvisable because it leaves you vulnerable to fornication in moments of weakness.
It also sets a bad example to others weaker than yourself and can bring the name of Christ into disrepute because most people would find it improbable that you are living together but not having sex.
My advice is to live apart until you are married to avoid these pitfalls and potential complications.
I hope you find this helpful.