Note from The Future: Of course women can wear tuxedos (and look amazing doing it) anywhere they like. And on some level Seras knows this. She just really wants to see Integra in a dress, and is jumping on this excuse like a cat on a laser pointer.
Integra: When will this “ball” take place?
Walter: A week from Saturday.
Integra: can we rent a tuxedo in time?
Seras: A TUXEDO?!
Seras: You can’t wear a tuxedo to a fancy ball, Sir Integra! Even though you wear men’s suits most of the time — it would be sacreligious!