Women simply don't waste their time following sports the way men do. A man will fly all the way to Las Vegas to watch a championship boxing match. If a woman did so, it would only be to accompany her boyfriend. On the flight out, rather than think about the fight, she would think about the luxury of her hotel room, the spa she planned to go to, and the celebrities she might see attending the match.
You never hear two women arguing passionately about, say, who the greatest female tennis player of all time was.
Go to any sports message board. It will be filled with guys arguing about which athlete was better in his prime, who is on steroids, which team will win, etc. Women are absent from those discussions.
A woman will never plan her afternoon or evening around a televised sporting event.
Women, even when they watch a sport, tend not to be knowledgable about the ins and outs of that sport, its rules and its history. They are more likely watching because they think the players are cute.
Women don't consider pennants appropriate interior decor. Nor do they consider jerseys with the number of their favorite player a positive fashion statement.
Talk to any bookie. All of their customers are men. Period.
How many famous sportswriters are women?
How many women go into coaching? Perhaps more to the point, how many of the women who do go into coaching are feminine? (That there are a few lesbians in a profession hardly negates the gender divide argument.)
Boys who run will pore over Track and Field News, comparing their times to those of runners around the country. Girls who run almost never do this.
The reason Sports Illustrated's bathing suit issue doesn't have any male models is simple: the magazine has no female subscribers.
There are exceptions to these rules, but they are just that.