Hi Sarah,
I've actually been wrestling with wanting to write a serious answer here, seeing as you ask a lot of really great questions regarding pop culture and how much we as Christians are allowed to participate in it without compromising our faith.
I tried to watch some of the videos I posted but found them to be a bit uncomfortable (for myself.) I think it was the first one that mentions Fritz the Cat -- ironically, I just saw a review about the animated movie a few weeks ago, and the fact that it was given an X rating -- and was all to proud of that fact.
To me, this kind of illustrates the thin line that's drawn when giving human-like qualities are given to animals. I grew up with cartoons like Bugs Bunny, but once they started featuring characters who were highly sexualized (like Babs Bunny in Space Jam,) it made me very uneasy.
Unfortunately, our human minds aren't very good at distinguishing lines. Now, am I trying to say that anyone who watches such things or participates in "furry" event is going to develop some kind of deviant attraction to animals? Certainly not. But I also have to wonder what we are doing to our minds if we're frequently watching characters that are drawn and styled to be part human and part animal while participating in very human-esque courting behaviors.
You probably know a lot more about this than I do, but I have read that one of reasons hentai was born was because Japanese artists started producing more and more explicit portrayals of humans with non-human or human-like creatures in order to produce animated pornography that bypassed the censorship rules (feel free to correct me if I'm getting this wrong.) My point in this is that there are very strong undertones of human-like relationships and sexuality behind these stories, which further inches closer to blurring lines.
I couldn't watch the second video I clicked on (hope I didn't get it wrong by mistake) because the narrator was making some quote about having sex with a dog? I know you had mentioned that furries see themselves as being completely separate from zoophiles.
I do wonder though how well people (and especially young people who are forming their identities) can keep the lines drawn in their minds. If they are constantly watching shows in which a "cute" "cat-girl" (a girl who is actually genetially part cat in the show) falls in love with a "handsome" "wolf-boy" (again, a boy who, in the story, is actually genetically part wolf), how long does it take before fantasy and a desire to bring that into reality?
And I'm certainly not saying that anyone with an interest in anime or furries will eventually have deviant thoughts about animals.
But the other things I think about are the genre of "pony play," in which grown adults are sexually attracted to dressing up and make-believing that they are actual horses, not only acting as if they are real horses, but only wanting significant others who act like other horses, horse owners, or jockeys (complete with all the equipment, such as crops and chaps.) So here we have an instance of humans who are mentally trying to be animals in order to be with other humans... who also mentally trying to make themselves as animal-like as possible (or part of that particular animal-centered community.)
I have no doubt that things such a the furry community can be -- not as an absolute, but as a strong correlation -- a springboard to these kinds of fixations and behaviors.
I am a girl who obviously spent some time liking cartoons (as if my avatar isn't proof enough.) But as with any genre, the boundaries are going to be pushed and eventually trampled, and so I would suggest that a fine line of discernment might be how well a person is able to make a clear distinction between their hobby (furry fandom) and real life (responsibilities, dying to self,) as well as keeping a lid on the not-so-wholesome roads one might be tempted to travel (i.e., avoiding Fritz the Cat.)
If a person is spending all their free time pretending to be an animal or to acting as if they have animal-like qualities, that just might be a big red flag.
