It's not discrimination, it's just preference.
Girls realise that it's not the guys choice that they're short and that they might be a really nice person regardless, however girls also cant help who they're naturally attracted to. Every girls preference is different, and it just so happens that the majority (definitely not all) are attracted to taller guys just because. The same way someone might be attracted to guys with black hair, or a guy who's tanned. There isn't a reason, thats just the way it is.. and I know it sucks for shorter guys but it's not like the whole female population hate short men.. women have to put up with the exact same things - eg boob size.