First off, there are plenty of girls in it. Second, they have good curves for young teenagers (14 or so), so they make great targets for hentai (hard to find a single series with more)--inevitably some will drift into the non-consensual category. Third, I think it's a fetish for Black Dog. He's done a tremendous number of SM doujin, almost all of them rape and/or submission. I think you can count the total number of non-SM doujin he's made with your fingers.
Seriously, those are some good lips. O_O
Cool to have this one in English, it's worth the translation.
Could it be because he is also getting old?
There's the reason.