Do you know why? Simple. Because God was there as a witness when you spoke your marriage vows to your young bride, and now you've broken those vows, broken the faith-bond with your vowed companion, your covenant wife.
God, not you, made marriage. His Spirit inhabits even the smallest details of marriage. And what does he want from marriage? Children of God, that's what. So guard the spirit of marriage within you. Don't cheat on your spouse.
"I hate divorce," says the God of Israel. God-of-the-Angel-Armies says, "I hate the violent dismembering of the 'one flesh' of marriage." So watch yourselves. Don't let your guard down. Don't cheat.