Oh what's that? Because you personally find it disgusting? How'd I know you were gonna say that?

Well I've got news for you about your personal opinions and the ethics of insisting that other people conform to them.

A lot of kink is disgusting to most people. That doesn't make it unethical. If someone likes getting peed on, or tasered, or verbally degraded because of their race (all things I've met people who are turned on by!), and they have a way to communicate to the person doing those things to them that they're not having fun anymore and want it to stop, and that person actually does stop, and does so immediately, there's no harm done.

No furry, no matter how depraved (and fewer furries than you'd think are horny for animal characters at all, much less quadrupedal ones), will ever actually fuck a dog, because real animals cannot consent and we know that. Furries who do actually fuck dogs get disavowed by the entire rest of the fandom almost immediately (see Hypnotist Sappho). This scenario is therefore entirely hypothetical, and if it's still morally wrong to you and you don't have an answer as to why, I think you have some work to do regarding prejudice.