What's really surprising is that he didn't get bucked in the face. Seriously, I own 4 horses and if they don't want you bothering them they will make it known. They will swat you away, kick, buck, head-butt, they're large and powerful animals. Even washing them, you have to be careful.
Now, aside from the fact that this is against nature and most major religion, chemically and medically it is dangerous to mix sexual fluids from two species. It can lead to a very high risk of penile cancer in some cases! And I think we all know how AIDS got started...
I always wondered about that. I always figured it was just dirty, disgusting & just wrong! I don't know, some people are sick.
That disgust is part of instinct. Its also the same drive that makes some of us straight folk sometimes queasy at the thought of gay sex. Your body's ultimate goal when calibrated specifically for genetic success is to find a mate and reproduce and care for family from your own species. We're all wired different though. Its easy to judge when you're normal. If I found out somebody was switched on by something weird, I'd reserve judgement based on their actions though because despite our drives we are NOT animals. Human beings are creatures capable (though not always willing) of superior reason and self restraint.