So people don't recognize...
...people by their voice? Ok, heights and builts could be similar, but the voice of a boy and a girl are DEFINITELY different from each other. Viola and Sebastian's voice were TOTALLY different, Viola didn't even sound like a male. Besides, their facial features weren't all that alike.
Ok fine, it's a movie.
Sir, I appreciate the struggle you have been through, and I am very sorry to have caused you pain.