There is love, devotion, respect, communication, confidentiality, and loyalty implied in the covenant of marriage. Why someone violates that trust, they violate another person. Adultery is not merely a sin against God, it is also a sin against another person. Additionally it is a sin against yourself. We do not live in isolation.
Adultery causes pain.
It destroys our relationships and robs us of our self-esteem. Adultery devastates whole families as it crushes our mate and cripples our children. It is for these reasons that God so opposes adultery. He gives us this Seventh Commandment for our own good. It is a negative statement that becomes a positive way of life for us if heeded.