"I've also heard it explained like this: Assume everyone wants to work (or talk about technical stuff) 95% of the time, and flirt 5% of the time. Assuming your company (or LUG) has a 1:10 female:male ratio, and furthermore postulate that all participants are hetrosexual. The women who do show up will be irritated, not because they don't want to flirt, but because they want to do other things as well."

Though humorous, this seems a surprisingly cogent explanation.