This piqued my curosity (not that I would ever do it) so I started reading. Apparently there isn't one piece of information available on the web that you can hold as truth. I read a couple of states action(s) on divorce and for each one that listed it's states laws on this, there was another to counter the law. Each state has various laws on the books and label them from "adulterous affections" to "lewd & lacivious behavior".
Some call them civil laws only, whereas other sites list them only as an affront to moral turpitude. I read portions that said every states has laws against adultery to sites that say no states have laws concerning it (as far a prosecution).
I guess the only way to know would be to consult an attorney...
Remember when you could get factual information from the web? I have now given up hope...