Shikamaru's parents have always taught him to respect the opposite gender. Not because of any perceived weakness or because they are lesser than men; no, he has been taught to respect girls simply because they aren't boys.

He likes to find the logical way of doing things. Logic is the best weapon and the best defense, a well-rounded fighting technique that demands the least possible exertion and tends to accomplish the task at hand quickly and efficiently.

A logical mind is a mixed blessing. Shikamaru sees things in the way a ninja ought, always understanding what is hidden underneath the underneath, but the same uncanny intelligence that makes him an enemy to be reckoned with on the battlefield often makes his personal life a living hell. He certainly doesn't miss Ino's occasional sidelong glances, nor is he oblivious to the delightfully painful thump of his heart whenever he sees her.

He really has no use for girls beyond a deep-rooted respect. Ino isn't exactly a girl in his eyes, though. She's a teammate.

Logically, that makes all the difference.