See one of the most recent cover re-imaginings at Jim Hines' site:
Jim C. Hines » Group Cover Pose Reveal!
"You can pretty much do anything if you can seduce the reader into it." -Ellen Kushner
That's a pretty amusing alternative cover shot, steerpike!
I put my hands up and say that if I am scanning over books or magazines (any genre) my attention will be caught by a beautiful female depicted therein. It doesn't mean I am looking for books illustrated in that way, but inevitably it is an effective way of giving me pause to at least consider reading the strap-line and blurb. I am equally likely to notice any other kind of striking artwork, from inanimate still-life compositions to dramatic design/logo renderings. If the tone, colours, images etc all conspire well enough together I will most probably take notice of their 'hey come over here' whisperings. I am highly visual, loving art as much as the written word. Millions of people are just the same.
The Old Testament has a fascinating account of mighty beings (generally understood to be angels but its a point of debate) that were so enamoured of the beauty of the daughters of men that they gave up their positions in the heavenly realm to take human wives here.
We might prefer that our literature be packaged differently, we might aspire to be less like moths drawn to flame, but the appeal of the human form is an ancient and deeply ingrained fact, and therefore a powerful marketing tool.
I did some googling and found some demographics for sci fi readers. (old article) I'll post the link because I don't really want to cut and paste a whole chapter, and hope I don't get in any troble with the mods.
Albert I. Berger Science-Fiction Fans in Socio-Economic Perspective: Factors in the Social Consciousness of a Genre
As you can see from these, the traditional demographic for sci fi has been male, and starting reader age, young, early teens and younger.
Is anyone attracted to specific cover artists? For instance, I can recognize Luis Royo's artwork and it always catches my eye. That might not translate to a sale, but it at least gets me to pick up a book and read the back cover.
Your target market, of course, doesn't have to be every fantasy reader out there, but you'd be smart to at least consider the recent trends in what people are reading.
For writers who want to appeal to a basically male (heterosexual) audience, the sexualised female on the cover probably does what it should.
But for writers who want cross-gender or female appeal, it would make sense not to assume a male (and heterosexual) gaze. The trouble is we're all used to the tradition, I suppose...
I can say for myself I don't go past sexualised females on covers all that often, unless there's a whole lot else happening. I do make an exception (when need be) though: Terry Pratchett, because he's so wonderfully humanist.