In general, I think the taunting rules are stupid. I guess they're supposed to protect the integrity of the NCAA's hallowed sport of college football, which is a joke in and of itself. But these aren't elementary school kids, they're legal adults playing one of the toughest sports on the planet. I think they can handle a little taunting.
All that said, the horns-down is-- by definition-- a taunt. Its sole purpose is to be disrespectful and mocking toward The University of Texas. So as long as the taunting rules are on the books, then horns-down should be called as an unsportsmanlike penalty. Same as "holstering the guns" or the throat-slash or any number of other gestures that have long been flagged for taunting by officials in NCAA football games.