I think while they may be a bit distasteful to some, you can’t really start considering unclothed animals as nude or most photos of them would be breaking the rules…
Still a blatant photo of animal’s junk can definitely be inappropriate and since you asked nicely I think it’s perfectly reasonable to mark NSFW (and also it’s a bit funny to do so too)
While the image made me laugh, I agree with you that it's not the subject (cat) that's offensive, but the blatant nature of the image. In a typical cat image, we don't normally have genitals as the focus, but this one is front and center.