most people who are inclined towards modern liberalism will not know how to answer this

I don't know about "most" people but I agree that it requires a specific post-modern liberal mindset to ponder this. Your other comment is the correct answer, and I think someone needs to be pretty far social left to even consider that lack of "consent" is the reason we don't fuck dogs. A right-wing person would have never even entertained that line of thought.