I would argue that the single biggest thing that undermines feminism are womens tabloid and fashion mags. They reinforce a body image that for many is unrealistic, with a focus on fad diets. They focus on appearance, celebrity, fashion. They reinforce the idea that its normal for a woman to go out the door with $10 worth of cosmetic goop on their face, when men don't have to. The content is mindless dribble with nothing truly educational or useful. Note how there is never, in a womens magazine, a science editor, a politics editor, a sports editor, a careers editor or a motoring editor. Magazines such as New Idea, Australian Womens Weekly, Who weekly are ideal vehicles to promote mainstream feminist values of equality and empowerment, however they just reinforce gender stereotypes issue after issue.
And guess what? They are mostly run by women.