I find this strange, because if I'm attracted to someone, this attraction doesn't change on a second-by-second basis, although perhaps its just me that feels like that.

Being attracted to someone and wanting to have sex with them next minute aren't the same thing. You usually want to also be horny to have sex. Women also want to feel comfort and trust.

A woman might feel: "I'm attracted to this guy but I'm menstruating and I don't like it to have sex while I'm menstruating."

where she might have a better idea of what she wants.

That assumes that a mental idea of what she wants drives her behavior. I think in most cases a woman will instead listen to her emotions that tell her what she likes in that particular moment instead of relying too much on mental concepts.

That desire might simply be: "I want to be more intimite with this guy than I'm at the moment but I don't want to be in public when we get more intimite."