While it's a bit of an extreme comparison, they're both weird, unprofessional imagery associated with things you wouldn't wan to associate with a software project.
I guess I'm not seeing the special category that an anime cat girl sits in. Is there some kind of sex implication I'm just not aware of? Linux has a penguin, FreeBSD has a devil(!), OpenBSD has a blowfish, Go has the weird Gopher thing, Gnome has a foot...
Wikipedia suggests that there's an association with queer and trans youth, is that what's meant to make the cock-and-balls comparison work? But it also says it has a history back to 17th century Japan...
If it were just the imagery I don't think this would be such a huge flashpoint relative to something like tux or octocat (the github mascot).