1 Corinthians 7:2
Nevertheless, [to avoid] fornication, let every man have his own wife, and let every woman have her own husband.
Note 4 at 1Co 7:2: From the context of this verse, it would have to be understood that Paul was saying that every man or woman who is not called to a life of celibacy should have his or her own mate. Unless otherwise called of God, everyone should be married.
Paul made it very clear in this verse that the reason for this is to avoid fornication (see note 2 at 1Co 5:1). Normal sexual drives are not sin. God created men and women with sexual desires that are meant to be enjoyed in marriage. So, failure to satisfy these desires can lead to lusts producing immorality (except for those whom God has called and given a special ability to be single).
Paul used this same reasoning again in 1Co 7:5 and instructed the Corinthians not to withhold the physical relationship from their mates unless it was with mutual consent for a short period of time. He said the reason for this was so that Satan wouldn't be able to tempt them through their lack of self-control.
The Apostle Paul acknowledged sexual desires as being normal and encouraged marriage as a deterrent to fornication. This by no means sanctions any form of fornication just because someone is not married.
Whatever situation we find ourselves in, God will give us the grace to conduct our lives in a godly manner. If we are not free to marry without committing adultery, this is no excuse for fornication. However, we were created for marriage, and it frustrates our natural, God-given desires to avoid it without being called to do so.