Matthew 5:27-28
“You have heard that it was said, ‘You shall not commit adultery.’ But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart. (Jesus is so counter-cultural. Many guys will say, 'I'm just looking; that doesn't hurt anyone,' but in God's eyes what we do with our hearts and bodies are much closer than we think.)
Hebrews 13:4
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 10:11-12
And he said to them, “Whoever divorces his wife and marries another commits adultery against her, and if she divorces her husband and marries another, she commits adultery.”
1 Corinthians 6:18
Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.