Premarital (aka extramarital) sex is fornication, which is sometimes differentiated from adultery. While adultery is specifically forbidden in the 10 Commandments, it would appear the Bible has the same opinion of fornication. There are times when the two terms are used interchangeably as well.
All from the KJV:
Acts 15:20 But that we write unto them, that they abstain from
pollutions of idols, and from fornication, and from things strangled,
and from blood.
1st Corinthians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
1st Corinthians 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
1st Thessalonians 4:3 For this is the will of God, even your sanctification, that ye should abstain from fornication:
So it looks safe to say that "Thou shall not commit adultery" also applies to fornication. The colloquialism "living in sin" is accurate.
It doesn't specifically say "Even if you plan to get married, fornication is still a sin." But the 1st Corinthians 7:2 reference is pretty close. To avoid fornication, get married.