Psalms 24:1-2 The earth is the Lord's and the fullness thereof; the world, and they that dwell therein. For he hath founded it upon the seas, and established it upon the waters.
God created sex not only for procreation, but to be an act of union and intimacy between a man and wife. God did not create sex for recreation. There are many reasons to abstain from sex outside of marriage. Some of which are to live without sin, to protect yourself from the possibility of creating a child outside of wedlock, protecting yourself from the acquisition of sexual transmitted diseases, and the creation of emotional attachments where there would not have been any otherwise.
One reason for abstinence that many people do not know about is that transfer of spirit that takes place during sexual intercourse. Sex is not just a physical act, but a spiritual one as well, even if there is no meaning or feeling involved. During sex, there is a transfer of spirit that shifts a part of your spirit to that other person, and shifts a part of that other person to you. This transfer is permanent, and yields a spiritual connection between the two of you. After this transfer, you are forever connected to that person, plus everyone that they have ever had sex with, because they are now a part of that person's spirit too. Dealing with all of these spiritual connections can be very difficult if you are a sensitive person.
Another thing to consider is that along with the transfer of spirit, there can also be a transfer of demons and incubus spirits. Many satanists use sex to acquire new members, transferring demons and incubus spirits to a person, in order to influence their behavior. Sexual behavior has been the biggest reason for the downfall of mankind, and their fall from the grace of God.