Why is banging your cousin even remotely acceptable in Japan? Come on, people.
Because at that degree of relation it's more of a social thing to dislike it than an actual potential problem like it would be if people more like siblings and parents were engaging in incest and reproducing as a result, probably?
I'm not gonna magically become attracted to any of my cousins just because it'd be more or less safe enough, since I'm already under the influence of a lifetime of that "cousins are off limits"/Sweet Home Alabama stereotyping and joking stuff. But I can understand why other places wouldn't have necessarily developed that same way on that matter. There's also probably some amount of holdover in some places from the "we must preserve our noble bloodline" mentality from ye olden times, where stuff like that mattered kind of a lot to a lot of people.
Anyway, in actual practice, I'm pretty sure a lot of Japanese people do think the cousin thing is weird. So even if it's legal they're not exactly likely to partake of it. I know, we've all seen plenty of manga where the parents of a male and female cousin are like "Haha, why don't you two just get married? We're not only cool with it, we'd welcome it." But how often is that really happening in real life? Probably about as often as this story's basic premise: a teenage boy holding off on his instincts to bury his bone in an all too willing attractive girl who practically throws herself at him.