It's not about sexism, it's about marketing. As long as sexy women sell magazines, they'll be on the covers. When they don't, be prepared for a rash of "kittenism" where social crusaders will turn their passion towards halting the exploitation of cute cuddly balls of furrrrrrr to sell their junk.
http://www.sbnation.com/golf/2015/4/2/8334591/golf-digest-lexi-thompson-cover-john-daly
"Golf Digest can't stop putting topless golfers on its covers"
"This is a cover, I guess. It has a woman wearing a towel, golfer Lexi Thompson, as the magazine's cover for their 'fitness and power' issue.
It is the second topless cover in a row for the magazine, as the editor points out here. (Rory McIlroy got the Green David treatment last month, or something like that.) That note hasn't stopped criticism of the cover for sexualizing yet another female athlete in a national publication, or for at least not giving men the same proportion of shameless golf-themed sexifying."