Fetishes are a symptom of evolution.
HOW they arrive a few posters here have speculated: early experience and wiring (or nurture and nature respectively, and both are happening and useful in slightly different ways). But WHY we have fetishes is because they propogate new genetic trends.
Fetishes (amongst all sexual preferences) are aquired by some random early experience, and encourage people to select a certain type of mate. By extension, though its important in its own right, these mates are more likely to belong to a single set (you only like chicks in glasses, say, and chicks in glasses get more attention from guys who like chicks in glasses). So, by seeking out in this way, loose groups (or networks) of particular pursued and particular pursuers form, having higher rates of interaction with each other and, inevitably, their young also (nurture). The principal still works if groups of fetishes overlap, and it doesn't matter if its your own biological child who is exposed to you.
As the pursuers in a group hold up such-a-type of 'model' and the pursued have that feature, their offspring will be more likely to have their random experiences centre around it. So the fetishes tend to be passed on, with those aiding survival propogating - socially - more successfully. Though fragile, traits aquired by experience can quickly adapt (see 'Meme'). Consider how the American model has gone from the 'fuller figure' of the early twentieth century lean times to the pencil thin as obesity becomes problematic. Models are just that: models. Across the century what's happened is that in most cases American women have simply tried to stay a little fatter or thinner depending on the vogue, to the betterment of their health (and so child-bearing abilities) in either case.
Wiring, or nature, is glacial compared to nurture and it records much more general things. To at least like those with everything in pretty much the right place probably does not qualify as extreme fetish, its just a useful bare-minimum to keep some genetic order. Things get hard-wired when groups gravitate for long enough around a successful feature. Then it becomes an advantage not just to luckily have the right random experiences to keep the trait going, but to have a literal kink in the brain that ensures that you and your offspring will do also. This too happens randomly, but it only needs to happen once in the right group to propogate further. It's also worth noting that nurture can over-ride nature: that's how the system changes over time.
Would that explain it? And take heart! A penchant for webbed-feet might seem freaky to them now, but if the global warming is true you might just be the next Abraham in a thousand years.
Me? I have enjoyed the sight of a pretty girl who knows how to smoke a cigarette... Guess I'm doomed to extinction, huh? :-(
