Take me with a grain of salt, but from what I understand Valve banned it when they received their evaluation build, and they found a scene of a child character "riding" on the back of an adult, naked, "horse" character, in the form of a human.

Developer immediately took to social media to whip up the current media frenzy you see now. Whether the developer simply didn't realize the implications of what they made, or whether they did it purposely for "artistic license" or "anti-puritan shock value" I leave to you.