There is a HUGH difference between going stag, or even going with a group of friends vs a girl showing up in a tux with her same-sex date or a guy showing up in a dress with his brokeback prom date.
Having said that, had she and her significant other been engaged in kissing, fondling on the dance floor, I would have immediately made her aware of the rules that SHOULD apply to ALL in attendance — it's a dance, not a bordello. You want to do that — get a room, but NOT HERE. My point is that this school district allowed itself to be used to further an agenda many of us find reprehensible and they keep doing it. This IS (for the short term) still America — your right to swing your fist ends at my nose. Cross that line, and you might find it uncomfortable.
If the district had simply allowed the girls to come to the prom, then the other kids would have shrugged the two weirdos off and gone on with their evening. I swear nothing more would have come of it (trust me, I remember how self-absorbed I was at that age — it's all about the dress and the date, baby!!!)