Exodus 20:14 - Thou shalt not commit adultery.
If you are not married to someone, you must practice abstinence. To have sex outside of marriage clouds your decision-making and gives you soul-ties that are not meant for your good. If you have been sinning in this manner, it is never too late to reclaim the purity that God intends for you to have.
Practicing abstinence outside of marriage protects you from a myriad of problems that you do not see coming. When you practice abstinence with intention, it makes you stronger, more focused on the things that are important to God, and helps you grow as a person.
Sex produces endorphins in your brain chemistry, much like drugs do, that lead to addiction of certain behaviors and poor decision making processes.
Any psychologist will tell you that drug addiction causes a stunt in emotional growth, and that when drug addicts stop taking the drugs for a length of time, their emotional growth begins again.
Anyone who has been abstinent for a long period of time who has experienced life on both sides of the sexual behavior barrier will tell you that abstinence has enabled them to see people for who they really are in a much clearer light; that dealing with problem people becomes much easier over time, and that you begin to practice much wiser discernment.