According to this site what really doesn't exist is an anime with a male character in it.
They seem to be frequent, but only because there are an ungodly number of submissions to danbooru. Think of it more like “for every male character, there are at least 10+ female characters.”
And it doesn’t help that a lot of the male characters are drawn in a female style...
https://i.imgur.com/9eO6ZpU.png [standard warning that it might contain nudity and violence exceeding your personal threshold]
It seems to me that if it truly used Danbooru, it also filtered for specific tags, which is also indicated by the art-style that it uses. The ratio of ostensibly male to female characters there is more so 7:18
There are tags for male/female which will be a better representation.
1girl: 3038k
1boy: 559k
Your idea about they filtered images could still be true, to be clear.
This explains a lot about why the generated images are the way they are (having a lot of borderline NSFW pictures and even if not having poses which are sexualized and/or imply sexual content).
While Anime tend to be more open to such things and tend to have often quite a bit of fan service the generated images are way to much biased compared to the "normal" anime/manga content you find on platform like crunchyroll or similar.
I mean a non negligible part of the pictures they used have likely hentai variations if you disable safe mode (If Danbooru is anyway similar to a site I used a view years ago to get nice screen background, if used in safe mode. And this seems to be the case.)...
But then due to practical (and maybe copyright) reasons it's very viable to use Danbooru as source but not so much to get snapshots of Anime's or pages from "real" Mangas.