Sexual sin includes not only sexual intercourse but also other sexual activities that are outside of God-approved marriage between a man and a woman. Sex is not only a matter for the body but also a matter of the mind.
The Bible reference
“…It is God’s will that you keep away from sexual sin as a mark of your devotion to him…”
(I Thessalonians 4:3 GNT)
In its proper context of marriage, sex is a good thing as the scripture stated,
“A man will leave his father and mother and be united to his wife, and they will become one flesh”
(Gen. 2:24 KJV)