I have often found the arguments about legislating against certain sex acts to be interesting. They are often quite convoluted and inconsistent and seem like cursory ways to push for a certain legal result rather than establish any clear ethics.
The “sapience” angle is sticky, because there are some who argue that even humans are hardly as self-aware as they suppose. It might explain away a human engaging in congress with a dog, but perhaps not with a dolphin. A dolphin lacks citizenship only for legal purposes, and arguably not because of its capacity as a rational actor.
Although I can appreciate the sentiment of protecting non-human animals, I think that that pretense is really only used to legislate away what many consider abhorrent human behavior. How can I claim that you violated a cow’s consent if you fuck it, but it’s then perfectly fine for you to kill it and eat it? There is also a strong element of “human exceptionism” here - why is it bestiality if a human fucks a sheep, but not if a dog does? Aren’t both instances equally “unnatural”? Does either demonstrably cause more harm to the sheep? This also frames the issue in terms of some vague “sexual gratification” - how about from an animal husbandry perspective? If I force breed or artificially inseminate some non-human, am I not still violating its consent, and arguably sexually assaulting it? If I forcibly inseminated human women, would it in turn be a valid defense that I did not consider the act to be sexual?
Then there’s the question of what agency a non-human can be said to have. If speech is required to establish sexual consent with a human, then doesn’t this also imply the troubling suggestion that non-humans are also unable to consent to sex with each other? How or why would this suddenly not be a problem? There’s also the problem of rationalizing away consent from a non-human who actively participates. If someone made a video of themself playing sexually, and then they were mounted by an uncoerced dog, would this imply they violated its consent? Would either the human or dog be unduly exploited if people watched the resulting video?
My point in pointing out these scenarios is that none of these factors are generally discussed when such laws are made, the underlying ethics are left vague and inconsistent. It’s an area where people seem to get quickly squicked so there is little debate. And as @Donald_Petersen alluded to, people are even quick to assume that anybody who wants to discuss the topic must have some personal stake in it, which is itself again weird and inconsistent. Why would debating the ethics of bestiality make one a pervert, while debates about, say, murder do not cause one to be equivocated with being a murderer?
tl:dr Since human exceptionism is an implicit presumption which makes all traditional law possible, people are reluctant to question it. And they like to police sex anyway.
