Two things:

First, if sexism is "behavior, conditions, or attitudes that foster stereotypes of social roles based on sex" (websters), then you are left with having female characters who cannot exhibit any behaviors or emotions that are stereotypically female. I am pretty sure no one would even make that movie, and even if it was made, then this website would probably just decry it as homophobic.

Second, if many of the films are sexist because of the views expressed by men, wouldn't that mean that the films are anti-men? If Seth is a clueless cretin who is scared of vaginas, shouldn't I be pissed off that adolescent males have been reduced to nothing but mindless bags of flesh that surround a phallus? If Vincent Gallo is an egomaniac who likes blowjobs, shouldn't that offend me as a non-egomaniac male who enjoys receiving oral sex? Should I fly off the handle because the male characters in Wedding Crashers are portrayed as shameless sex-hounds that will go to extraordinary lengths to trick scores of women into sleeping with them. If portraying someone in a way that they exhibit behavior that is in line with gender stereotypes is sexist, then I would defy anyone to come up with a movie that is not sexist, and I would posit that any movie that is sexist because a man views women as needy, neurotic, emotional, solely a sex object, etc. is more anti-men than anti-women.