$5 million, but bestiality becomes the most common fetish.

* Much like incest is now, it becomes the "fun in fiction, not in real life" kink that a lot of people gravitate toward. It remains illegal in real life and nobody would actually touch an animal who wouldn't already.

* It displaces rape and incest as the kinks which people gratuitously insert into stories whenever they can. Instead of a comic using rape to show how evil a villain is, they show off a character's depravity by depicting them shacking up with animals. Instead of a novel using a brother-sister romance as the tender taboo, it's now between a boy and his dog or a girl and her dog.

Sounds absolutely hilarious. No drawback.

It doesn't have to become your fetish, just the most common one.

Deal.

Things like incest fantasy are mostly just in porn, as are most hardcore rape fantasy stuff. If beastiality stays illegal then it won't even be able to take over the mainstream of porn sites I use.
I've only ever known one chick who was too into a rape fetish for me to the point it ended the relationship, I imagine this wouldn't actually be a problem in practice with most women.

And if the rape thing is replaced then that might mean some rapists end up raping animals instead of people, which don't get me wrong is still fucked up, but ultimately not as bad as raping another person.

Doesn't seem like there's many downsides.

It only becomes common in fiction. It doesn't affect real life.