Let's think about the 7th commandment of God's Ten Commandments; You shall not commit Adultery.
The strength of any society or nation can be seen in whether we are able to keep the commandments of God. It is very obvious in our society in this generation that many people are not keeping this commandment. To commit adultery is to be sexually unfaithful to your wife or husband.
However, many people do not understand the spirituality of God's holy Law. Our Lord Jesus Christ said in the Sermon on the Mount in Matthew 5, verse 27 "You have heard that it was said to those of old, 'You shall not commit adultery." "But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart." Jesus is saying that you can break this commandment in your heart many times by looking lustfully, and yet no one might know except God. But God does know, and so He marks it down in his book of judgment, and unless you repent, change your mind about it, and turn away from it, He will bring it up with you on the Day of Judgment when He will judge all the people whom He has made.
Why? Because marriage is intended by God to be exclusive to one man and one woman for life. Therefore, any sexual activity outside of marriage is sinful in His sight. God's intention in commanding this is for your good and your protection. It is so that you will find this greatest of earthly joys and pleasures in a holy way, giving yourself to that person whom you will marry, in a way that will bring glory to God, and bring you to a greater understanding of what God knows will bring you true joy and fulfillment.
He is not merely trying to keep something from you, which is good. God's intention is that sexual activity be governed by purity in your heart and mind so that you will truly know how to give yourself in love, to that one special person of the opposite sex whom He has given to you. Your being pure in mind, and heart, and body is found in waiting to unwrap this good gift of His until He says it is the right time. He has stated in the Bible that the right time is marriage.
If you are living together with someone outside of marriage it is called fornication in the Bible. Hebrews 13: 4 says: "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge."
What then can be done to change; to please God and find your greatest fulfillment? It is to realize that you must believe in Jesus Christ, the only pure and perfect One, to be saved from your sins. When you believe in Him, He will give you grace to change, and joy in keeping God's commands.