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I am sorry, I do not have good news. Apparently watching or reading incest porn is not against any laws - state or federal - providing there is no child porn involved. If any of the porn is involving or exploiting a minor child.
If your husband is watching incest videos that include minor children I would strongly advise you to go to your local police and report this. Not only is it against the law but it is a known fact that people that watch child porn are much more likely to offend a child.
If this is not a child porn issue, then by law there is nothing you can do to stop it. You have the right to file for divorce if this is a major issue in your marriage, with his actions most Judges would feel you have a justifiable issue for leaving. If you have young children in the home, you may want to consider their safety in the home as well.
I wish you the best