Personally, I think it’s a gender-based difference. Please understand that I’m about to speak in very broad generalities, and as such, individuals may vary.
To ensure maximum reproduction, men are “programmed” to be ready to mate at the drop of a hat (with as many women as possible, since their reproductive cost is relatively low). Because of this, they are often primarily stimulated by the visual. If a new possible mate enters the area, they have to be ready immediately, or risk that some other man will inseminate her. I think that for a man, sexual attraction is often fairly immediate, as they have to make that spot-decision.
On the other hand, if a woman mated willy-nilly (heh!) with just any man she came into contact with (as would be a man’s strategy), it would serve her poorly. A woman has a greater personal cost involved with reproduction, as it’s her body being taken over for nine months (and care for long after); she might even die in childbirth. It befits her to be far more conservative and choose the best possible mate out of the selection of willing men sure to present themselves as suitors. You can’t always make this decision lightly, quickly, or based on looks alone – it takes a woman a little longer to warm up, I think. As such, she might not be immediately attracted to a mate, but it can grow over time as his fitness as a potential father is measured and evaluated. It’s not always based on looks, although appearance helps a lot.
To review:
MEN—> Can get off merely at the image of a woman in a Vicky’s Secret catalog.
WOMEN—> Usually require a couple dates, a personality, emotional attachment, and possibly status and resources to be attracted to a guy.
Again, not true in all circumstances, as there are sensitive men and slutty women out there.