I would argue that a “Chick flick” is a movie that is designed and marketed specifically to appeal to female moviegoers.
“Thelma and Louise” was not a true Chick Flick; while it appealed to women and was about women, it was more of a Kick Ass Bitches Movie - Note the prominent use of guns and a big explosion in the trailers - in the same group as “Crouching Tiger, Hidden Dragon” or “Aliens,” which is a different kind of movie that invites men to enjoy lots of high-quality ass-kicking, the twist being that it’s chicks who are kicking the asses. (Actually, Thelma and Louise was about half Kick Ass Bitches Movie, and half Standard American Road Movie.)
“Sisterhood of the Travelling Pants” is a true Chick Flick, since it’s clear that from a marketing standpoint they aren’t even pretending to try to entice men to see it; the film is exclusively aimed at women, and if some drag their boyfriends along that’s just gravy.
Consequently, I would have to argue that “Fever Pitch” is not a Chick Flick. While it was primarily about romance and relationships, the film clearly was marketed to both men and women. Note that much of the second half of the trailer is baseball scenes, such as crowds cheering or David Ortiz smiting a home run. That’s clearly meant to excite male viewers.