Don't know what all the fuss is about this.
Ayatollah Khomeini clarified the issue for moslems:
"One of things which will cause an animal to be Haram, although it was originally Halal, is a human having sex with it and ... because of this work (sodomizing the animal) its meat and the meat of its lamb which will be born after sodomizing will all be Haram, along with its milk, wool, and hair.
If the animal which someone had sex with is an edible animal like a sheep, cow, or camel it should be killed (Zebh) and burned. If it is one of the animals which is not usually eaten but is used for riding and transportation, like a horse or donkey, it should be taken out of the city and sold in another city."
It does not say how many times the horse or donkey can be used before it must be sold.
This must be happening often enough in moslem countries for him to give an opinion on the subject.