If a man is found lying with the wife of another man, both of them shall die, the man who lay with the woman, and the woman. So you shall purge the evil from Israel.
Adultery was a serious sin in ancient Israel... much more serious than we take it in contemporary society. The standard of God's Law for Israel was death for the offenders. Both parties caught in adultery were to be put to death. Capital punishment for marital infidelity was the requirement of the law. God sees adultery in the same class of sins as rape, murder, idolatry, and willful rebellion. All of these were punishable by death.
Why does the term "capital adultery" seem ludicrous, perhaps even barbaric to our ears today? It is because we have a very cheap view of marriage and an even lower view of the holiness of God. We place personal sexual satisfaction on the throne of our hearts and have it dictate our view of human relationships. Understood, instead, as Deuteronomy describes it, adultery is a way to kill a marriage, and that is why God listed it as an offense that carried the death penalty. He did not want His people to be casual, lust-filled, cold-blooded marriage murderers. He wanted them to purge the evil from them.
God created marriage. He blessed it, and the sexual relationship within marriage. He made it the foundational element of human society, forming from it both family and home. It is the first human relationship into which we were meant to come into this world. Christian marriage pictures the gospel, a holy and amazing thing full of grace and truth. Marriage has always been as sacred as worship. It is sacred like our very lives are sacred... to be dedicated to the holiness and glory of God. That is why degrading it with sexual sin is wrong. To do so is to shun God and all that is holy.
Now I thank God that the blood of Jesus covers even the worst sin. Even the sin of adultery. And I have had the privilege of seeing couples find forgiveness in the grace of Christ for this, proof to me of the sufficiency of Jesus' work on the cross, so that their marriage did not die, despite the near mortal wounds of an adultery. If anything, though, the fact that it took the cross of Jesus to atone for this is breathtaking in its sorrow, and cause to keep a high view of Christian marriage. Yes, there is forgiveness and grace even beyond an infidelity. I will always fight for that when a marriage is in crisis. I will give them the hope of new life in Christ and the resurrection of their home with repentance, faith, forgiveness and grace. But I'd still have to say it: adultery is a marriage killer and only Jesus can resurrect murdered marriages with His life.