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.
Might not be their cat - however, if it is, you absolutely need to get it fixed for a myriad of reasons. If you need resources, hmu, I can help you find vouchers or local programs to assist with spay/neuter.
Why so hostile? It is in the owner and the animals best interest . The cat has less health risk, is more docile, and won't create more strays. The owner (if it has one) get a more chill cat that won't mark everything it sees.
Spay and Neuter is necessary to control an already booming stray cat issue in the us. TNR programs help communities and keep feral cat population down without needing to euthanize cats to control their spread.