I guess the furries don't feel the need to create alt accounts for their porn

yeah, makes sense since so much anime content (or at least, a signifigant proportion more compared to western animation) already includes pretty explicit content. The line between something like Kill La Kill and kill la kill R34 is already razor thin.

part of it is also community. From a quick glance I recognize that r/hentai is in anime island instead of R34. But the moderation of r/hentai has (or used to have) many moderators from other anime subs, and some regular posters will alternate between r/hentai, r/manga, or r/animemes. That can play a part in which communities decide to form and comingle.

I feel like moderators should receive a weighted vote when identifying connections between subreddits. Two subs with identical moderator lists must be pretty similar right?

That doesn’t make sense when you realize mods uphold arbitrary politics of a sub, often even more than they are just janitors.

Same reason why there's a "gay island" I guess?