We know that whenever the Bible refers to fornication, or sexual sin, premarital sex is included in that. Why? Because of the Old Testament. Premarital sex was actually punishable by stoning to death. So the audience knew full well that premarital sex was sexual sin, and so should we.
Through Christ we are liberated from the old ceremonial laws. But God's moral laws are eternal, and that includes the sanctity of sex within marriage. If we are filled with the Holy Spirit, we are not simply free to sin as we choose. We are free from the sin, as the Holy Spirit leads us towards obedience and glorification of our Lord. When we belong to Christ, we are members of His body, not enslaved by selfish desires and sinful lusts, but instead our lives become focused on serving Him. We actually gain the desire to leave sinful ways behind us.
Let me ask you this. What do you think sex is? Why do you think God created it? What is its purpose?
If sex is what actually consummates the marriage and makes the two become one, what is it outside of marriage? We know that the Bible tells us if a man has sex with a prostitute, the two become one flesh. In essence he has married her, right? How is this different than any other kind of premarital or extramarital sex?
We know that adultery is sex outside of marriage, right? So if a man has sex with someone other than his wife it is adultery. What if he is not married? Then he can't really have sex with his wife, right? Because he doesn't have a wife. Sex with anyone when he is not married is adultery because he is not married to her. This is a really simple concept.