I found this and it pretty much explains it:
The law against adultery used to be more vigorously enforced in the days before most states adopted no fault divorces in the 1970's. Prior to no fault, adultery was used as grounds for a divorce. Proving adultery may have been difficult for a wronged spouse, but being arrested for violating this law was a difficult thing to explain away at a divorce proceeding.
Today, with all states granting no fault divorces, most government authorities view adultery as private business. When marriages may unravel, the state gets involved as the arbiter of divorce. In these cases adultery may be considered in determining the split of marital assets, or child custody. Adultery may be considered as an additional crime when a person is caught and charged. In these ways, the state still takes an interest, without having to take on the role of policing the bedroom. Besides, they already have their hands full with policing the streets.
Just under half of the states in the United States still have an adultery law on their books. The states with no adultery laws have nothing to enforce, but the ones that do have these laws clearly do at least have something that they are able to enforce, if they so choose.
