Legal Question in Family Law in Texas

It probably isn't. Ask him to stop.

If there is a "morality clause" in your final decree, then let him know that he's violating that clause by having the GF in the house overnight when the child is visiting. Otherwise, if you don't have a morality clause, you're either going to have to work something out with your ex OR sue him for a modification and ask the court to make additional orders for the protection and well-being of your child.