If a zipper zipped down it seems to me that you would be in more danger of getting something caught in the zipper and if your fly was not zipped (it does happen from time to time) for the horse to get out of the barn.
Chronos - yeah, I’ll grant you that. I’ve had zippers of that type fail that way, which is, perhaps, why you see the big-toothed metal jobs on pants more often. Although I thought of it because the pair of shorts I was wearing when posting that had the nylon type on them.
Why do they need to be straight?
Maybe some sort of U-turn at the top (making it shaped like an upside-down J) would provide a place to “park” the zipper without having to worry about gravity. I’m not sure how tight a turn a zipper cn handle, but it’s a thought.
Screw it: wear a kilt.
Incidentally, if you have a pair of pants with a “magic fly” – one that all too often succumbs to gravity – the trick is to put the metal ring from a key ring through the hole on the tab of the zipper, and hook that over the button when you zip up. This requires a zip-then-button habit rather than a button-then-zip habit.
– Dragonblink, who wears her jeans to threads
Dantheman are you Irish by any chance?
No offense there, Dragonblink, but I think I’d look pretty silly with a keyring protruding from my groin. 
Uh… why, Griffin?
And I thought we were talking about pants. 
Button flies. The BEST thing since flares.
No, no. Button flies are a tool of the devil. YOU try unbuttoning one of those mofos when you really, really, really, really, really, really, really, really, really, really, really, really, gotta go. Yikes!
Or if you’re drunk off your ass. Double yikes!
Yep, that’d be a banner day to wear the ol’ Depends…
Personally, I don’t think having someone unzip your zipper in an upward direction would be nearly as erotic as feeling them slowly easing your zipper downward.
Uh, if you’re zipping up, you can see what’s about to be ‘zipped’. If you’re zipping down, you’re zipping into uncharted waters, so to speak.