“You shall not commit adultery.”
God created marriage as something special. He made it as a covenant between a man and a woman. He made it to be lasting. Any activity outside of the marriage covenant is outside of what God intended and will lead to problems.
There are numerous programs that are trying to prevent STD’s and teenage pregnancy. Unfortunately this is just treating an effect of what happens when we go outside of God’s plan for marriage. And it doesn’t treat other effects such as the emotional baggage that is carried from one sexual relationship to another.
God didn’t decide that we should only have one partner in order to kill our fun. Instead God wanted us to experience sex to its fullest which can only be had within a monogamous marriage relationship. Study after study has shown that it is not the people who have many partners who enjoy sex the most but those who keep sex within the confines of marriage.
Like idolatry, adultery usually begins with small compromises that can be justified easily enough. Most people don’t meet someone new and immediately think that they want to cheat on their spouse with that person. It slips in gradually as one thing leads to another. However, the consequences can be dire. The only way to be certain to avoid them is to not make the small compromises in the beginning.