Apparently it's him liking NSFW of an underage character(that's been aged up, a cheap-ass cop out), and that's all I really know.
Can anyone fill me in?
Did Ninjamuffin know about that character being canonically underage, or did they just find that art piece and enjoy it, without being aware of the character's age?
ThylaTank
Considering Ninjamuffin's situation, it's relatively tamer when compared to Softdon(Cassete Girl being based off one of his SA victims).
it's still shitty.