There are any of a number of reasons why an accent might trigger feelings of sexual interest. The phenomenon is very well known, and has been treated in many different tales. Dracula, for example, combined the dual Victorian paranoias of disease following sexual contact (syphilis was the AIDS of its day) but also the fear that good looking foreign men would come and steal the virtue of their womenfolk (it should be noted that middle and upper class Englishmen were often sent on a tour of Europe, in part, to lose their own virginity before marriage to a woman whose chastity wouldn't matter.)
It's possible that the accent evokes a romantic notion of a place never met. Paris, for example, is seen by many as baguettes, great music, culture, fine food and coffee, and suchlike, and a Parisian accent to a girl from Enumclaw might make her see the world for a bit with very rose coloured glasses. She certainly wouldn't be seeing Paris as the Frenchman does - the above delights the tourists, but the truth of living there sometimes includes alleyways that reek of urine, no place to park, ever, a frighteningly expensive cost of living and the reality of a rather grim graffiti-spattered banlieue where people get robbed at knifepoint. Not to beat up on Paris - but everyone has a romantic notion of what another country will be like to visit, versus actually being there.
It can also work in the inverse if said country is not exactly in vogue. An American accent in Paris was certainly not welcome in the early 2000s just after the initial invasion of Iraq.
But the core reason why the sound of a voice clearly indicating that it is from somewhere else might make a girl (or a boy - it works both ways) melt might bypass the higher functions completely and go straight to our evolutionary history.
Recent research shows that we pair bond as much on scent as we do on reasons we can intellectually understand. And that by scent, we're referring to subtle cues in the scent a body gives off that speaks to its health, its immune system, and its compatibility, genetically, with the person smelling it. This has been recently implicated in the phenomenon of "gaydar", as well.
And when you look at it, evolutionarily, the purpose behind any courting ritual or mating is to produce healthy, viable offspring.
One of the keys to successful sexual reproduction is to find someone whose genes are sufficiently removed from yours to avoid some pretty bad combinations. When a gene pool in an area gets sufficiently shallow, such as the grim industrial Mancunian slum Moss Side, or European royalty, health issues and birth defects skyrocket. In fact, some cultures, such as Iceland, have changed their mores to allow married women to mate with complete strangers with the blessing of their husbands, in order to improve the gene pool of the area.
And a really big obvious clue indicating "I am not from around these here parts", and therefore "I am in possession of different genes" is an accent. Speaking as such to powerful evolutionary drives, wired that far back in the brain - it's no wonder that it's a powerful draw for members of the opposite sex.