Looking on a woman lustfully is adulterous
"You have heard that it was said, ‘Do not commit adultery.’ But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart" (Matthew 5:27-28).
Put to death sexual immorality
"Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry. Because of these, the wrath of God is coming. You used to walk in these ways, in the life you once lived" (Colossians 3:5-7).
Sexual sins come from the heart
"For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander. These are what make a man ‘unclean’; but eating with unwashed hands does not make him ‘unclean'" (Matthew 115:19-20).
Sexual immorality, including premarital sex (fornication), is described in detail as sin against God and your body, the temple of the Holy Spirit (1 Corinthians 6:12-20)
The sexually immoral will not inherit the kingdom of God
"Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God" (1 Corinthians 6:9-10).
God can cleanse you from the sin of sexual immorality
"And that is what some of you were. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God" (1 Corinthians 6:11).
Jesus can set you free (John 8:31-36)
Purify yourself out of reverence for God
"Since we have these promises, dear friends, let us purify ourselves from everything that contaminates body and spirit, perfecting holiness out of reverence for God" (2 Corinthians 7:1).