Perhaps some of the only porn someone can consume ethically. I guess there's an argument to be made about poorly paid animators drawing smut in animation workhouses, but it feels like most of the western world, when it comes to hentai, consumes rule 34 of franchise characters from media. Beat my brain up as I may, I can't see anything wrong in that and would much rather a fictional character be debased than an actual human for money.
Couldn't the writers just have sex (with other people)?
Whether the artists could make their own reference material is a different matter from whether they do