In an interview Katy Perry said that this song isn't based on an actual event; rather, it's an expression of her admiration of timelessly beautiful women.
And to most of the other people who commented on this song: you don't have to be bisexual or homosexual to like it! Cor! Stop being so freakin homophobic! We don't care if you're bi! It doesn't change your value as a person! @^$^#*