Spoiler: toady's comic
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I'm guessing Nanase isn't aware of how hyper-seductive she seems to be, and that such seeming is why Ashley is reluctant. Which seems fitting with what Grace noted int he link Dan made in the comments.

I find it hard to imagine that Nanase isn't aware of how she seems... but such appears likely as I doubt she'd be intending to act thus and, as noted with Grace, she thinks it's more cool than sexy.