I'll just post a few verses from the Bible which make it very clear to me (there are more). You interpret them how you will.
Matthew 5:28 ESV
But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
1 Corinthians 6:18 ESV
Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.
Galatians 5:16 ESV
But I say, walk by the Spirit, and you will not gratify the desires of the flesh.
1 Thessalonians 4:3-5 ESV
For this is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God;
Colossians 3:5 ESV
Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry.
1 Corinthians 6:13 ESV
“Food is meant for the stomach and the stomach for food”—and God will destroy both one and the other. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body.
Galatians 5:19-21 ESV
Now the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God.
1 Corinthians 6:9-10 ESV
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality, nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God.
Hebrews 13:4 ESV
Let marriage be held in honor among all, and let the marriage bed be undefiled, for God will judge the sexually immoral and adulterous.
Mark 7:20-23 ESV
And he said, “What comes out of a person is what defiles him. For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person.”