seriously

It's not accepted, but you can marry your cousin, if i'm not wrong. The thing is that incest is a fetish so it makes it interesting, I guess?

Of course it isn't. Readers aren't reading this because they want an incestuous relationship with anyone in their families. In fact, if you ask, most of them would react with visceral disgust at the idea. Kinks and fetishes are always about the forbidden elements of sex. Gay sex isn't necessarily "the love which dares not speak its name" anymore, so a significant thrill of danger has dissipated from yaoi stories. That thrill has been a large part of the attraction of romance for a fair number of readers, who develop an emotional correlation with their own sexuality. They simply want to be loved and desired so much that their beloved risks everything to be with them, even social approbation and censure. Incest stories are a type of cypher for that force of attraction.

I never said people who read this wanted to have incest relationships. I said it was a fetish, which can be real of fantasied. It doesn't have to be real to someone liking the fact that certain thing is happening. And, as you said, fetish create a fixation over an object or situation making it sexually attractive, as in incest. But it doesn't mean that people have an actual sexual attraction over the thing they have as a fetish, they can just have a catharsis with this kind of stories and it still would count as one. The fact that people read this stories to see something that would never happen and imagine what would be like and release strong/repressed emotions is the definition of catharsis, and if they are enjoying it because it has a certain aspect, as incest, is what we usually call a fetish.

And I was responding to the OP's comment, not yours.

Incest as in cousins relationship's are legal in Japan and in many other countries. However, any other incest relationship are illegal in Japan.

Thank you