Transcription:
How To Confound A Centaur
Centaur: Hold it right there, you can't just ride through my fields uncontested! I'll lose my credibility if we don't battle or something.
Me: That's fair. How about we–
Centaur: And it can't be a pun battle, I heard what you did to the sphinx.
Me: Darn. Okay, what if I beat you in a horse race?
Centaur: Ha! Alright, your funeral. Where's the finish line?
Me: That tree over there. Where's your horse?
Centaur: (gestures at horsey backside) Um...?
Me: You're not a horse, this is a HORSE race. You have to race with an actual horse.
Centaur: You want me... a centaur... to RIDE a regular horse.
Me: That is, linguistically-speaking, what you agreed to.
Centaur: ...
Me: ...
Centaur: I hate you.
Me: That's fair.
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Is it gross if a centaur borks a regular horse?
And if the answer to that is yes...is it more or less gross than if the centaur borked a human?
Way less gross. Human and centaur are both intelligent, can communicate, and give consent, so it would be fine. With a horse (which has none of these things) the centaur would be committing bestiality.
Well, in many games there is a Speak With Animals spells or equivalent. They are capable of communication, we just usually can't understand.