She was invited to the prom under the rules that you propose and her response was to sue them. Obviously, she preferred her sin to their company. It was she that ostracized them, and not the other way. They merely gave her what she said she wanted, but decided not to have any part of it, since it would be condoning her lifestyle.
There is nothing morally wrong with freedom of association, which includes the freedom not to associate.
Snakey wanted “one more chance” for her, making a gesture that predictably would have been spurned. One does not throw pearls before swine.