With that said, there is a definite line of intelligence that animals don't have. Fish can't speak. Birds don't make telescopes. Dogs can't do your taxes.
Animals have, well, animal instincts. It's that simple mind that, when it wants something, it goes and gets it. When a dog is curious, it sniffs things. When it's horny, it humps your leg. The whole idea of social constructs and things being inappropriate is simply incomprehensible because a dog is simply not smart enough to understand it. In fact, babies and small children (and sometime full-grown adults) have this same condition. But where a human's intelligence generally grows tremendously to be able to learn all this stuff, an animal's maximum capacity isn't that high.
Animal intelligence is an interesting concept to play with. How do we treat the same level intelligence differently? A 12-year-old dog and a 3-year-old child have roughly the same intelligence, but do we treat them the same way? If a 30-year-old has a traumatic brain injury that gives him the mental age of a child, do we treat him the same way that we treat a child of that actual age? And again, why?