It's a result of the masculinity dominance that has ruled our culture for centuries
Personally I tend to disagree with statements like this ... to me, many men's lifestyle magazines put females on their covers because it's a business move to attract men, who have higher sex drives, to buy them. Female magazines, on the other hand, put females on the cover not because they feel submissive to men (as is somewhat suggested by the statement above), but rather because they subconsciously want to look like the women on the covers. To me, the male psyche works on a slightly sexual basis ("I think she's a really hawt babe and I'd love a girlfriend like her") whereas the female psyche works on a more cooperative one ("Perhaps I can learn something from this really stylish woman so I can make myself look even even better"). A lot of people claim that females want to look better so they can get a boyfriend, but I think it's a sweeping statement because many women want the
confidence achieved from looking good and not the romantic benefits it may bestow them.
Plus, it depends a good deal on what the magazine is about, and its target audience and selling method. Magazines like FHM or Maxim are a lot about satiating the beastly male appetite (LOL), but there are other magazines like Men's Health and International Vogue Hommes that don't have such an aim. The former for example puts pictures of males with buff bods on the cover because it appeals to the "I want to look like THAT!" mentality (not unlike that used by female fashion magazines). The latter puts pictures of stylish men because again, it is appealing to the "I want to look as stylish as THAT!" mentality.
LilyTiger: I know of perfectly straight men who buy magazines on style such as Men's Folio (an Asian publication on menswear fashion). Men's Folio seldom shows females on the cover; the content includes menswear editorials, articles on the latest menswear collections, and some other things appealing to men e.g. models (cars, that is

), expensive pens and stationery, watches, and so on.