The people who think incest porn should be banned are loud and proud in their beliefs. They’ll put up posters, tell their MPs, respond to surveys, and appear in political debates.The people who support incest porn are a lot less talkative.
I think there is an argument to made the pornography in general is harmful.
But to single out one single type of porn strikes me as... very odd. Maybe politicians can list, explicitly, all the other porn genres they find acceptable or agreeable to them, as a kind of compare and contrast exercise.