Is Shikamaru sexist? or is he just old fasioned? Well, neither, but both to be quite frank. I mean, he does seem to have some old fashioned ideas about fight girls (as in he doesn't really like hitting them and when he fight he typically makes them fight themselves. ^_^) but he is also sort of sexist in the fact that he can't lose against a girl.
All in all, it's probably Ino's fault as he doesn't really like her very much. ^_^ Well, he does (at least fans speculate) but that's another matter.
His distaste for girls isn't really a problem though. He doesn't go out of his way to avoid girls, or go out of his way to make girls lives miserable. He just doesn't enjoy their company. ^_^;; It's hard to explain . One could argue that he is disrespectful to woman, but he's really disrespectful to everyone. It's mainly strong willed girls that extremely beautiful that he has problems with such as Tsunada and Ino.
Furthermore, he does state that he wants to be married to an average girl of average looks. So he's not gay (as i suspected).