I have indeed observed this trait over the years, but had never thought it worth a mention on a forum.
As I once caught the contents of a little box of drawing pins in this way, I feel that wimmin may well be right in this instance. 
Glad to have my "research" confirmed .... I too have regretted the natural impulse to close my legs when dropping something .... in my case a lit cigarette .... which is why I made no jokes like "surely you are used to a little prick between your legs".
However to get serious again... Gerlies are trained from an early age by their mothers to "sit properly, knees together, never cross your legs, only your ankles .... 'in case you show your knickers'" whereas boys and men sprawl, legs akimbo without a care in the world. So why the change when they drop something?
These same little gerls will happily do handstands against the school wall, perform cartwheels etc without a care in the world who sees their knickers but heaven help any boy who tries to "cop a peek" under the desk. My two were terrified of picking up a dropped pencil for fear of being accused of "inappropriate behaviour".
In my younger days we always threw a ciggy to a girl in the sure and certain knowledge that we would be granted a brief glimpse of stocking top at the very least.
I tried this out on my ex, many times, when she was sitting at her dressing table, nakkid and she still reacted the same way, by opening her legs but if she walked up stairs in front of me I was accused of lechery.
Conclusion .... Wimmin are weird.
