@givemersspls He's just an elementary school brat. He wouldn't be quoting law books. He would have just heard a friend or some older person say you can't marry a cousin (sound advice), and he would then spread the info, true or not. That's how kids operate.
@givemersspls It doesn't take more than a single person. Might have even been the TV and a foreign show. In my opinion it's not a flaw in what I said because he did issue such a claim, so it ought to be what he believes. He had some reason to believe it. It's not like elementary school boys would be much imagining their future marriage anyway, so they most likely wouldn't start the discussion on such a thing, meaning the information they gain would depend on random events. Other than that, with 120 million people in Japan, there would be quite a few who would think marrying a cousin isn't the wisest thing to do. Might even be his parents. For a kid "You shouldn't marry your cousin" == "You can't marry your cousin".