Where does the argument stop? Why limit yourself to vertebrates, or even animals? Is it cruel to poison an ant nest in your yard? Is it cruel to eat wheat, cut down at the prime of its life? What about corn, which wouldn't exist without human intervention? Is sex with a flower, a plant sex organ, count as bestiality?
Humans can empathize with things which aren't humans, and with animals is relatively easy to empathize with pain. The laws are a function of, among other things, our level of empathy with a species and its use for other means. Dogs and cats, beloved by many, take a very special place in our laws, while tasty chickens are less so. Lowly wheat is neutral on the empathy scale, while most are decidedly not fond of roaches.
The balance of concerns changes over time. Should some day we be able to synthesize meat directly ("Panda meat while you wait!" "Put some tiger in your tummy today!"), then I predict there will be a larger call for "more ethical" treatment of animals currently raised for food.
The arguments against bestiality vary. Some just think it's squicky, which is likely an aspect of empathy. Others think that in human cultures, sex must be consensual (not rape), by people who can make an informed decision (not children, not unconscious), and without the taint of a power imbalance (boss/employee, teacher/student). Though even then, an adult child having sex with a parent is also squicky by many despite passing these rules.
Empathy with animals extends that view to animals, and it's hard to tell if an animal consents, and is not doing so from power imbalance.
I am not an ethicist, but I think this gives an outline of the factors by which we tend to say that sex with delicious pigs is a no-no while sex with pretty flowers is odd, but not specifically regulated.