Girls put a lot of effort into their hair. I've dated over three women so I can tell you this with some authority. I can also tell you that women have what's commonly referred to as a "bad hair day" where their hair is not cooperating. As such they quickly throw it up in a pony tail.
Other girls notice and understand the reason, guys notice and wish more girls wore their hair up. Seriously! Pony tails are sexy. They show off a good neck line and make us imagine that maybe you're athletic. I say the more pony tails the better. Let's start a movement.