Are you sure? I've seen many women being fucked by dogs etc. but no men. Also it was discovered in research that women get more excited than men watching other animals mate. I believe that the right situation with the right state of mind (hormones etc. you know how women get affected by all that shit in their brain?) women can get turned on by just about anything.
The reason why, is the doggy,horsey shit is a man's fantasy...but, more men are more willing to engaged either being screwed or screwing the pooch, so to speak...go take stroll at some of those forums...and you'll see that bbers are normal compared to those fuckers. There was a youtube vid of a couple who was screwing a shetland pony...the guy didn't think he was gay having a male horse screw him.
There is also a cave painting in which you can clearly see the intention of one man upon a horse...
Basically, man will fuck anything...