Found this article while doing research for a presentation at work. It's an excerpt from a book called Dressing with Dignity by Colleen Hammond, published by TAN Books. . Most of it is a review of women's fashions, starting with Adam and Eve and ending with the effect on trousers in marketing.
It seems that when women wear trousers, men's eyes immediately follow the line up their legs to the inevitable "woman's most private and intimate area. Not her face! Not her chest!". Apparently "Men's eyes will follow the lines right up her legs and finish the picture in their imagination. Women's eyes may do the same thing, but since women don't have the same type of temptations, their imaginations don't complete the picture in the same way as men's do."
This is my absolute favourite quote out of the article:
"When a woman is wearing pants, a man's eyes will (much to his embarrassment) fall to a woman's crotch. These men also pointed out that it is something that happens without their wanting to do it, or without their realizing it. It's the nature of men "to look" ... and they do!"