A fetish is not the same thing as having a favorite sexy body part. Certainly not boobs. Women have boobs because we were selectively bred to have them by generations of men liking them. Why else are we the only primate, or really the only
mammal to have large, decorative breasts? (Udders don't count)
It is completely natural to like boobs--and butts, since they are another secondary sexual characteristic that we have specifically to attract men--and it is also natural to enjoy clothing and linerie that accentuates these important secondary sexual characteristics. Thongs, leggings, and jeans all show off a nice, round butt. And DUH--revealing clothing is GOING TO BE EXCITING because it shows more of the good stuff!
Fetishes are something that go a little outside the norm, that involve something that, biologically, isn't part of the usual constellation of body parts and behaviors that attract men and women together.
Mini-skirts with an inch of butt showing? Not a fetish. Bunny costume? Fetish. Bustier that shapes a narrow waist and makes the hips and breasts look larger (hourglass figure)? NOT a fetish. Black leather mask? Fetish!
That being said, you can learn a lot about fetishes here:
http://www.avclub.com/content/savagelove
This is a well-written and respected sex column specializing in the most unusual of fetishes, and he often addresses issues such as where they come from or what to do about it, or how to tell your girlfriend. The most common causes seem to be sexualized fear (beating fetishes, for example) and power (diaper fetishes, for example) or a combination (rape fetish).read the column, it's useful for the vanilla types too.