My understanding of it is a bit different. The ranch owners were unaware of what was happening. Apparently their employee was surreptitiously taking gay men to the ranch to be on the receiving end of sex with horses. He filmed the encounters and sold the video on the internet.
I was in Arizona when the story broke and read about it in the papers down there. It said that there were all different types of animals being used, and harnesses set up for the specific purpose of man/animal sex. They also said that the ranch drew guests from all over the country, like a dude ranch.