Minorin had her future long and carefully planned. In Japan, getting married pretty much means you're retiring (though this is recently shifting).
Minorin wants to play baseball just as Sumire wants to be astronaut. A romance interferes with that goal.
So basically, Minorin would be giving up her dream of pro sports for Ryuji.
Watch a documentary called
[Senkyo] Campaign about a guy running for office in Japan - his wife is also a career person (they're quite modern) and behold the *pressure* she gets from the campaign managers to retire and be a "proper housewife". It was filmed recently (2005).
trailer:
http://www.youtube.com/watch?v=UJYUG1o22DI
review:
http://www.popmatters.com/pm/review/...mpaign-senkyo/
My impression is that the author of Toradora! (a female) wanted to write a more complex romance in which the women were having to make hard choices themselves - she intended on shaking the male readers into some wakefulness about these creatures they chase.