You know that makes you a dirty little pervert, don't you, kristie?
Coming from the one who posted in it like five times!
Are you kidding?! The man would never be able to move. Have you ever seen a mouse's testicles? A mouse the size of a golf ball has testicles the size of marbles. No joke.
He is a typical male human except for three equine-like features- hooves, a tail, and genitals (no, Not that big). So in other words, sexually he is a horse...
Then I must've had some rather well-endowed mice. They were probably the envy of all their locker room pals. And that would explain why all the little girl mice fainted dead away whenever they were around.
(I'm joking about that last part. I didn't have female mice.)
Oh, wait: You want a novel where people with various horse-parts boink (repeatedly, in a variety of positions)?
Here ya go: World Enough and Time by James Kahn. I read it years and years ago.
Ideas ... anyone can have 'em and they're all equally good. Writing the book, now... not so easy.
The difference is that animals are not deemed capable of giving consent--thus is is a form of abuse.