You do make a sane point about cruelty.
That might apply to smaller animals like chickens, but surely not to donkeys. Have you seen the size of a donkey's wang? You think a human wang is going to hurt a lady donkey? HAH!
And even so, is this any crueller than things that are already legal, such as branding cattle, de-beaking hens, etc?
Informed consent? Do we require informed consent from racehorses?
Ability to refuse? Well, you've got me there. From now on, my position is that you're not allowed to shag any animal except horses, and even then, you must respect the principle that neigh means neigh.