Fever Pitch (2005)

When Lindsey Meeks (Drew Barrymore) a big time executive meets funny and charming high school teacher Ben Wrightman (Jimmy Fallon) they hit it off until Lindsey realizes Ben is a Red Sox fan. She herself must realize the Red Sox or Ben before she breaks it off and can accept his obsession with the Red Sox.

See: Fever Pitch (2005 film) at Wikipedia