Yes, sex is for married people only. Otherwise it is fornication, another sin. Engaging in repeated sin is a stronghold, and a sign that a person doesn't know God, according to scripture.
Romans 6:1-14 is important!
Sin’s Power Is Broken
 1 Well then, should we keep on sinning so that  God can show us more and more of his wonderful grace? 2 Of course not! Since we have  died to sin, how can we continue to live in it? 3 Or have you forgotten that when we were joined  with Christ Jesus in baptism, we joined him in his death? 4 For we died and were buried  with Christ by baptism. And just as Christ was raised from the dead by  the glorious power of the Father, now we also may live new lives. 5 Since we have been united  with him in his death, we will also be raised to life as he was. 6 We know that our old sinful  selves were crucified with Christ so that sin might lose its power in  our lives. We are no longer slaves to sin. 7 For when we died with Christ we were set free  from the power of sin. 8  And since we died with Christ, we know we will also live with him. 9 We are sure of this because  Christ was raised from the dead, and he will never die again. Death no  longer has any power over him.  When he died, he died once to break the power of sin. But now that he  lives, he lives for the glory of God. 11 So you also should consider yourselves to be  dead to the power of sin and alive to God through Christ Jesus.
 12 Do not let sin control the  way you live; do not give in to sinful desires. 13 Do not let any part of your  body become an instrument of evil to serve sin. Instead, give  yourselves completely to God, for you were dead, but now you have new  life. So use your whole body as an instrument to do what is right for  the glory of God. 14 Sin  is no longer your master, for you no longer live under the requirements  of the law. Instead, you live under the freedom of God’s grace.