Sexualizing animals is unethical because animals do not have human intelligence and therefore cannot give consent. The entire premise of the furry fandom is "what if animals had human intelligence and did human things?"

In this fictionalized world where animals walk on two legs, wear clothes, talk to their friends in English, and stress about how they're going to pay for their apartment, and where, often as not, humans as such do not exist (for an example of such a world, please see Zootopia, Fantastic Mr Fox, etc.), it's not really out of line to draw a buff two-legged wolf man, give him the gruffest possible voice and make an animation where he talks dirty to you, and then go "man, I wanna have sex with that guy".

The important concept here is fiction, and being able to distinguish between it and reality. No one's going to suggest you're a criminal because you went on a killing spree in a videogame. In real life, where animals do not have human intelligence, furries react to people who want to boink actual animals the same way most people react to people who want to boink children, and for pretty much the same reason.