I have no evidence, outside the authority of Jesus, why fornication is wrong.
The Spirit calls fornication and other similar things immoral.
"But to this day whenever Moses is read, a veil lies over their heart. But whenever a man turns to the Lord, the veil is taken away. Now the Lord is the Spirit: and where the Spirit of the Lord is, there is liberty
But we all, with unveiled face beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit.
And even if our gospel is veiled, it is veiled to those who are perishing. In whose case the god of this world has blinded the minds of the unbelieving, that they might not see the light of the gospel of the glory of Christ, who is the image of God.
2 Corinthians 3 - 4
The Lord is the Spirit and the Spirit gives life. Jesus is the life. To go to the Father you must go through Jesus, who is our life. "I am the truth, the way, and the life, all who to go the Father must go through me." Jesus is our life and we share in His life. Jesus is in us and we are in Jesus. He dwells within us, we are a Holy Temple of God, in which the Spirit dwells. We are called to live Holy and blameless before God. Jesus can't deny himself, therefore he is always faithful. Our life is hidden in Christ. We must live with the Spirit, who is the Lord, and in step with the Spirit. This same Spirit was sealed in us when we believed in the gospel preached to us. Christ is God's glory and our life is hidden with that glory. Since we share ourselves with Christ, we share in his glory, and we must live for the glory of God, doing the will of the Father, who is in Heaven."