Mhm, because beastiality and zoophilia are sexual attraction to NON-human animals, and yiff is human-like animals since furries are half-human and half-animal
I mean, I'm fine with yiff I just don't enjoy it myself, you feel me?
Can't say the same for zoophilia tho, those people can go to hell(the ones that can resist their urges can stay but go***mn they need help)