Just John said:
Would you please explain this "arrow effect" thingy to me so I know what I'm not supposed to notice?
Supposedly, the shape of pants forms an arrow pointing up to the crotch, but the fabric in a skirt does not. I've seen a couple illustrated versions, usually showing the upside-down V shape formed by the legs as an arrow pointing up, but one of them claimed the arrow was made by the outline where the legs and trunk came together (if you have defined lines there, your pants are way too tight).
I just assumed it was more common than it seems to be, because I've joked about it with others who were raised similar to me and most of us had either seen one of the pics or heard it used as a sermon illustration. The whole effect is just as present when a man wears pants, but some people seem to forget or overlook that women can be pretty visual too. That said, it stuck with me enough that I still try to wear tops that are long enough to cover that whole area when I do wear pants because I now have the image burned into my head.