Every guy may have his specific
girl-next-door prototype, but in general, she'
s the
girl whom you always admired from afar and were afraid to approach, fearing that any erotic projection toward her would ruin her image as a decent, pure and almost virginal womanly ideal.
She's (in your mind at least) untrodden ground; unspoiled by other men and so
sweet-natured it almost frightens you to think of her in explicit sexual situations. Almost. Here's another example: Picture your sister's
best friend from when you were kids. In your mind she'll always be that 10-year-old
cutie who played with you in the park. Only, when you catch up with her 10 years later, she's still a
cutie, but all grown up and someone you can trust and connect with on so many new levels. The intrigue grows.
Similarly, she could be a friend's cousin whom you met at family occasions over the years. Or even a new employee at work who is shy and unassuming, but still gives you that "she seems so nice, I'd love to talk to her" feeling.