They need a reason in order to maybe convince normal people that they're not attracted to dogs and kids so that they're not seen as a danger to society? To also question their own sanity for wanting to have sex with something that looks or acts like a dog or a child? The reason is also needed because they specifically choose to mimick sexual acts with something or someone that by default cannot consent. If they can't pick any of the trillions of things to mimic during sex that are consensual, then they need a reason to justify that.

Again, if you want to pretend to be subhuman, then do that. Why does it have to involve pets or children? It's an evasive explanation. Why not pretend to be a non-existing alien species?

Sexual involvement makes it different because if you want to be a dog, then the worst thing that can happen in that you just bark at my dog and he will look confused. If you combine sexuality with dogs, then my dog might have to suffer through a lot worse than a weirdo barking at them. The moment you involve sexuality, consent becomes extremely important. And when your sexuality is centered around mimicking something that doesn't possess consent, it gets worrysome to me.