When you have more than one partner, then you are in the act of cross contamination. Just like in fast food, if you use the spoon for more than one dish, you are putting the people at risk of cross contamination. It's the same with the human body.
When people are participation in fornication and adultery, they are cross contaminating.
It's God commandment to only be with the woman you marry, and to not put her away. He calls it a sin. Jesus said that if a man commits fornication, he is defiling he's own body.
God gave us the commandments for our own good, not to hurt us.