“And this second thing you do. You cover the Lord’s altar with tears, with weeping and groaning because he no longer regards the offering or accepts it with favor from your hand. But you say, ‘Why does he not?’ Because the Lord was witness between you and the wife of your youth, to whom you have been faithless, though she is your companion and your wife by covenant. Did he not make them one, with a portion of the Spirit in their union? And what was the one God seeking? Godly offspring. So guard yourselves in your spirit, and let none of you be faithless to the wife of your youth. ‘For the man who does not love his wife but divorces her, says the Lord, the God of Israel, covers his garment with violence, says the Lord of hosts. So guard yourselves in your spirit, and do not be faithless.’”
Malachi 2:13-16
How often do we cry out to the Lord, wondering why he doesn't answer our tears, prayers, and groanings? The prophets of Israel offer a glimpse into why God's voice was silent under the old covenant, and I believe that the reasons largely remain the same today. However, when you move the perspective into the new covenant, where Jesus increases our responsibilities, suddenly it becomes clear why many of the offerings of God's people are not regarded. If looking at another woman with lustful intent is now the same as the act of adultery, brothers, are we being faithful with our eyes?
It's not just important for the health of your marriage relationship, for your own purity, and for obedience to God's will... It's also important because without purity of our eyes, mind, and heart—facilitating an unhindered union with our spouse—God will not accept or regard our offerings of prayer, thanksgiving, and praise.
“So guard yourselves in your spirit, and do not be faithless.”
Secondly, Malachi goes on to mention what God is looking for out of your marriage union: Godly offspring. If we are not faithful to our wives in body and mind, we cannot be completely one with them. When you are not completely unified as a couple, it is much harder (read: impossible) to raise your children in godliness.
Furthermore, as the leader in your home, your moral failures have a direct, negative impact on your children's spiritual wellbeing. I think the clearest illustration of this is David's failure with Bathsheba. Not only did God take the child which was born out of wedlock, but you can see a rapid decline of morality in his entire household. First Amnon defiled his sister Tamar, then, as prophesied would happen by the prophet Nathan, (2 Samuel 12:11) Absalom defiled the 10 concubines which his father left behind to keep the house.