14 Thou shalt not commit adultery.
Another very brief commandment, this one forbidding married individuals from breaking their marriage covenant and having sexual relations with anyone other than their spouse. Of course, this one commandment does not cover the entire breadth of sexual sin. It does not mention anything about fornication, or bestiality, or incest, or homosexuality. However, just because all these matters are not covered in the ten commandments does not mean that they are not covered elsewhere in the word of God. Here in the mountain Moses received the foundational rules of God’s law, but there were other visits to the mountain, and other details yet to come. Leviticus, for example, has several chapters that cover the other sexual perversions that are expressly forbidden by God.
I think it is fair to say, though, that adultery is the root evil that all other sexual perversions are an extension of, and this explains why it would be forbidden first in the ten commandments. Calling out adultery points to the fact that sexuality is fundamental to the union between a man and a woman in the covenant of marriage, and so it is the perversion of that order that constitutes a sin. Whether we also pervert that order by having sexual relations before we are married, or with people that we cannot be married to, or with animals, it all follows the idea of breaking apart the marriage covenant and taking the things which belongs to it elsewhere. All sexual sin is in the spirit of adultery.