Porn sites are full of incest and people blame Japan for being weird. It's just a common trope.
Yeah but porn is porn. The criticism isn't about porn. Lots of crazy shit is done in porn that I wouldn't want in my average fiction. I'm surprised I have to say that porn isn't a good barometer.
They are right about it being a common trope though. Just look at the romance genre as a whole, there's so many stories about incest, both contemporary and ancient.
Oedipus Rex by Sophocles, Pericles by Shakespeare, Ada by Nabokov, to name a few. The list goes on