I loved Natsuki but even though this is a yuri series and I have my yuri goggles cranked up to the max I still didn't expect that flare-up of yuri subtext when Natsuki thought "I wish I could've gotten you excited about softball instead" or something like that.
That is the core of my "Natsuki might be gay or bi" sentiment. I don't think she was head over heels for Yuu or anything, but it does seem to suggest that she wishes that she could've been more special to her.
I don't think there's any particular reason to argue that Natsuki HAS to be gay, but I think there's enough there that it wouldn't be out of left field, either.
Unless Natsuki is an outfielder.
You know, you people shipping Natsuki and Sayaka are being kind of stereotyping by assuming that every short-haired, sporty girl in YagaKimi is a lesbian . . . wait, I’ll come in again . . .
Hey now, we're (by which I mean I'm) assuming that one out of three short-haired, sporty girls in YagaKimi is a possible lesbian. The one that visibly pined after her best friend's affections after (unknowingly) listen to her gripe about her girlfriend. I don't think that's too unreasonable. ;p