Dinosaur Erotica: Acceptable Beastiality

I strongly doubt this is a joke.  The short story (this is not a novel) is written seriously and with some serious choices which are based on scientific theory: the T-Rex has feathers, for example, alluding to theories that the dinosaurs evolved into birds.  However, the fact that humans and dinosaurs coexisted is, of course, a fantasy created for the sake of the erotica genre.  

Beast erotica is a legitimate fetish, and roleplayers pop up on reddit's DPP at least once a day looking to be fucked by a fantastical beast.

Therein lies the catch:  according to pornography laws in the US and the UK, bestiality is not permitted.  But monster porn/erotica is.  The monsters have to be mythical or ancient creatures: unicorns, dinosaurs, dragons, hydras, etc.  They cannot be animals that live in current times.

I fail to understand this distinction, and, when I'm in a good mood, I find it utterly hilarious.  What is the core of the objection to bestiality?  The cross species sexual contact?  Why would that be improper?  Because of the possibility of breeding, of creating human/animal hybrids (even though this isn't possible)?  Is it based on Christian theology and the idea that sex should only be between a man and wife (yes, homosexuality and other sexual orientations not allowed)?

Personally I worry about animal cruelty, but that's not an argument I hear very much against bestiality.  In fact, I hear the opposite.

When I was teaching Theatre History at a university in Texas, a student came up to me after class and said he had a problem: a guy he had a crush on was in a sexual relationship with a golden retriever.  "I really like this guy though." 

I swallowed and asked, "Is the dog being hurt?"

"No!  He loves it.  He's always on this guy humping his leg."

"Perhaps," I said and took a deep breath, "you should look at this in terms of a human relationship.  If this guy was fucking his roommate, would you be okay about it?  Would you want to be with him?"

"No."

"There you go."

And there I went: judging and arguing that bestiality is a sexual orientation, and once which should be seen as identical to a sexual relationship between two humans.