Why isn’t Adultery illegal in the US?

Adultery ( cheating on your spouse ) should be illegal in a marriage. Marriage is a contract where both agree to stay monogamous and not to sleep with others. If one doesn't want the marriage anymore and doesn't want to be with the other person, then leave. But cheating does immense harm in a marriage. It creates complexities around divorces, destroys families, the father not knowing who the kid is from, the kid growing up all over the place, and many more emotional and financial issues. It shouldn't be legal even if the other consented, because marriage needs both sides' approval as well. If a spouse can sleep with others without needing the other spouse's approval, it implies the marriage is still 2 individuals instead of a union. When you're not married, you can cheat from a legal perspective. Marriage is fundamentally different because both parties have a right to the other. Cheating is never not wrong but in marriage it causes devastating losses outside emotional. This is why adultery, for both sides, should be illegal and lead to jail time. This incentivizes people to only marry if they agree to stay monogamous.