Several gaming groups I've been in have dubbed this the 'Law of inverse feminine armor class'. Indeed the notion seems so pervasive that I was regarded strangely for creating a female NPC who actually does wear armor! These groups have even gone so far as to devise house rules for this, such as stating that any female not wearing armor gets their Charisma bonus to AC (I hate this one personally), or that a bikini made of any armor material provides equivalent protection to the full suit. No, neither of these really make sense.
Equally annoying, from my point of view, is armor with the 'Cleavage++' special ability, that is, armor crafted in such a way as to show as much cleavage as possible. Of course, this uncovers one of the areas that armor is most meant to shield! I personally don't use these rules, and I appreciate the fact that the core D&D books, for the most part, depict their female characters as being appropriately dressed, including full armor where appropriate. Honestly, some of us who play RPGs actually want to roleplay of all things, not just be part of some bogus, gimpy fantasy that's more about T&A than getting into character or even monster-slaying.