There is an implied a fortiori here. If weed is illegal, how much more so should sex with horses be illegal. Because sex with horses is worse than marijuana. This slips in the unexamined assumption that marijuana is pretty safe.
Also, why is having sex with horses a bad thing? If it is bad because it is cruel to the animal, that is illegal in all fifty states and DC. Not having a specific bestiality law doesn’t make it a legal practice.
h/t Everitt Mickey and Eric Gustafson