The real question in regard to prohibitions on bestiality is what kind of person commits bestiality? Anti-social people. There are a number of studies that offer evidence that bestiality is fundamentally an erotization of control, violence, and objectification, and that these zoophiles rarely have just one paraphilia. Investigators in Sweden noted how often zoophiles got busted for CP, IIRC around 35% of them, which is a disproportionate number. One survey of zoophile forums shows that 75% of zoophiles are willing to break the law to engage in bestiality. Think about the kind of person who has no qualms about breaking laws to sate their desires, and you'll obviously start thinking about rapists and pedophiles, same category of people. Another study shows that, among convicts (convenience sample, sure, but it still matters) those who engaged in bestiality were significantly more likely to commit interpersonal and violent crimes compared to the non-bestiality group. The FBI has looked at the backgrounds of serial killers and discovered high rates of bestiality. In studies of juveniles sent to juvenile detention and psychiatric centers, it was noted that those who engaged in bestiality were more likely to have committed violent crimes and disproportionately likely to suffer from increased numbers of mental illnesses compared to the others.
Point is, there's a real reason to prohibit this kind of person from indulging in their erotization of violence: We want the state to be able to get these freaks into psychiatric wards before they can commit atrocities. I can post the studies if anyone wants them but I'm too lazy to sperg right now and flip through the data. Maybe I'll put them all in a post here at some point, but they're not hard to find. The fact that animals are suffering is almost a secondary concern, and reasoning from that basis is full of logical traps that stump people all the time.
Then we can appeal to other consequentialist arguments; for example, society doesn't gain anything from bestiality, but it stands to be substantially harmed through unnecessary risk of novel zoonoses. One could also argue that, even the argument that zoophilia could be prophylactic for rapists, allowing them to substitute animals for humans, fails on account of the fact that these people are practically training themselves to be better rapists, which we obviously wouldn't want. They are testing their techniques on animals and desensitizing themselves to the act.
The fact that animals cannot consent to sex still matters though, even if we view it in an anthropocentric way. Zoophiles often hold that bestiality is a kind of affection they give to sexually-deprived animals who they anthropomorphize as having human faculties and feelings; but there is obvious difficulty in determining whether an animal consents. Since they cannot speak and we can't yet read minds, assuming animals even have an intelligible mind to begin with, we will never understand unambiguously what an animal intends through its actions. Just like pedophiles often mistake innocent gestures by kids, like a smile or laugh, as "consent," how do we know zoophiles aren't mistaking an animal's gestures for consent when it's actually unrelated? We will also never know whether an animal is silently suffering, unable to express really how it feels after suffering a lifetime of sexual abuse.
This ambiguity itself cannot be overcome through measuring stress response, because all objective indicators of animal stress are ambiguous and only reliably measure excitement both positive and negative, not stress. Namely, the measuring of glucocorticoids (there's no correlation between glucocorticoid levels and stress in general.) An important problem in using glucocorticoids as an objective indicator of stress response to a specific event is that it's actually very hard to get an accurate measurement of glucocorticoid levels proximate to the event, since all methods of measuring it either cannot be taken on command (urine, feces,) themselves inherently cause stress (blood, saliva,) or accumulate glucocorticoids over a long period of time (hair follicles,) and all methods are subject to a high degree of variation, not only because glucocorticoid levels fluctuate in an unstable way over the course of the day, but also because chronic illnesses can alter levels of the chemical, and natural variation can occur for no perceptible reason. This is similarly problematic in humans too, by the way, attempts to measure the neurochemistry of stress. Animals react in varied ways to stress, sometimes they will even be friendly toward their abuser, so there's no way to tell if an animal is being harmed from an act at all. It's completely black-box.
Therefore, we cannot ever tell if an animal consents to something. Zoos often argue that an animal can be trained to consent, because there are examples of horses or dogs being trained to use symbols or sound boards to indicate what they want, like food or a blanket or to be let outside, but this has an obvious ethical issue: If a zoo wanted to train an animal to indicate whether it wants sex, the zoo would have to rape the animal as part of the training process, violating consent in the first place and poisoning all subsequent "signs of consent." And this is, of course, also complicated by the fact that, as I just established, animal intent behind behaviors is opaque and inscrutable to us. And a further consideration: How would we know that an animal's consent is actually intentional consent and not just Pavlovian conditioning? That's something crucial to establish, because if it's just conditioning, how would an animal conditioned to have sex be any different from a sex slave?
So in the end, there's no argument they can even make that bestiality could ever be consensual until we develop mindreading technology or something, which we do not have. fMRIs are notoriously opaque, and as I mentioned, neurochemistry is very opaque as well. Science just can't read brains yet, if it ever will.
If zoophiles, in response, want to jettison the idea of consent altogether, then they really don't have a justification that they're anything other than sexual predators. If something can't consent and you have sex with it, you're having sex without mutual assent -- without consent -- which is rape and would be as unacceptable if done to a human. They will have to argue either that non-consensual sex is morally unobjectionable, which is approval of rape, or they will have to claim that animal consent is separate from human consent, in which case they demolish their argument that animals are similar to humans and on that basis they can justify treating them as humans (which they don't do anyway, they confine them in houses like pets just like everyone else.) So essentially, they either approve of the sexual objectification of animals (predatory,) or they destroy the basis of their entire argument. Lose/lose.
Whether we should apply the standard of consent universally to all treatment of animals is kind of a moot point, since a lot of zoophiles will accept the idea of human consent and apply it to animals. If they do that, they're caught in the contradictions I just mentioned. The ones who don't anthropomorphize animals are basically approving of rape in a transparent way that everyone can understand. But fundamentally, the whole telos of this argumentation is anthropocentric, since the main question is whether the knowing demolition of animal consent echoes morally against human consent, which I've shown that it does. We can leave the matter of the meat industry/chattel pets/livestock aside and just settle for pointing out the contradictions.
Finally I want to point out that it's perfectly reasonable to ban something on the grounds that it's just naturally disgusting to most people. Obscenity and/or indency laws are about as old as law itself and are even today used to juristically uphold prohibitions against CP and certain kinds of fetish content. Even bans on sodomy are still used in some jurisdictions, for example, in the US Military, to prohibit male rape. We ban the obscene to maintain societal coherence and solidarity, and to avoid bringing harm upon those who might be susceptible to being unbalanced by seeing certain things. The extent to which you think that should be done is up to you, I prefer having intellectual freedom as much as possible so I'm generally against most kinds of censorship, but this could still be a perfectly legitimate justification against bestiality.
And also we cannot forget teleology, which has been used in the past to prohibit all kinds of behavior as unnatural, since it doesn't befit the final cause of humans, which in a sexual sense is to reproduce, to create and involve all the inner sexual principles in an act, and form a union between the masculine and feminine. I prefer this route exclusively since it allows us to stay consistent with the self-evident fact that animals and other things on the earth exist for our utility while avoiding gross sexual disrestraint, but it's obviously going to be a tough sell in secular modernity.