The issue comes down to an issue of authority.
Does the individual decide what is right or wrong, or does God?
God has said in his word that premarital sex is wrong, Jesus said if you look on a woman to lust after her in your heart, you have committed adultery already in your heart.
The problem is that God created men and woman to marry and to be in a permanent relationship so that they could raise godly children, premarital sex is wicked mainly because it is rebellion against what God says is right or wrong, but some other reasons would be that it contradicts the family structure that God has ordained, Also kids growing up without both a mom and dad in the same home has devastating consequences on the kids, I know because I am one of them, and it took the grace of God to change my life and get me heading in the right direction. God has ordained that sex be something reserved for marriage.
But really the issue comes down to who is your final authority, you and your feelings, sexual impulses, and desires, or God.
That is the basic problem every human being must face, it is the reason why people embrace atheism and why they reject God, because people do not want to submit to the authority of God, we are all rebels, we have all rejected God, Romans 3:23 for all have sinned... We have all lied, cheated, stolen, hated someone, had lust in our heart, and done wicked ungodly things that we knew were wrong when we did them, yet man loves his sin so much that he tries to find intellectual arguments and reasons to ease his conscience and make himself feel better. One day all will stand before God and all will be guilty of sin, your only hope is that you repent of your sins and turn to Jesus Christ, he died on the cross for the penalty of your sin. Romans 10:13- for whosoever shall call upon the name of the Lord shall be saved. John 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish but have everlasting life.
My suggestion for all reading this is, do not seek for ways to justify your sin, go to God and admit that the sin you love is wicked, and confess your sinfulness to Christ, and ask him to save you, and he will cleanse you of your sins and change your life. That is what happened to me, I was a drug addict and a fornicator and I always was seeking ways to justify my sin, but we all know deep down inside that we are wrong. You know the truth, For those who have ears to hear, Hear me, God is warning you, Repent from your sins and turn to Christ now while you still can, What shall it profit a man, if he shall gain the whole world, and yet lose his own soul? Why do ye labor for that which satisfies not? Jesus told a woman at the well, whoever drinks of this water shall thirst again, but whoever drinks of the water that I give shall never thirst. How has the waters of sin satisfied you? it always leaves you thirsty, there is living water. I'm sure many will mock at this message, but God is speaking to someone, you know if he's speaking to you, listen to his voice.