The dress codes I've seen have been pretty specific on what men should wear (i.e., slacks not jeans, dress shirts, etc.). Then again, I've never seen one that bothered to forbid dresses or skirts, and what isn't expressly forbidden...
What about men in kilts?
I second Andrea. Most I've encountered are not necessarily prohibitive of dresses.
I have a feeling there is a good story behind this question.