The main reason the horses can't come in the house is, they won't stay on the ground floor, of course, so then they'll go upstairs (quite the operation in itself) and, no matter how careful and considerate they are, you'll hear KLOMP KLOMP KLOMP all afternoon. I don't want to even contemplate how they'd come down the stairs so they can go back out to the barn. It's really better for all concerned if they stay outside. You're quite lucky that they let you come visit them in their "home."