There aren't really hard and fast rules for the gender of sunglasses, but some stylings tend towards "female" or "male".
As a first pass, women's sunglasses are larger and more decorated than men's. For example, this is a random pair of men's Versace sunglasses (on the first page of Sunglass Hut's results for men's Versace sunglasses):and this is a pair of women's Versace sunglasses (similar query, s/men/women):
I don't think I've ever seen the medusa on any of the men's sunglasses.
Women's sunglasses also tend towards being rounder or curvier than men's sunglasses (reflecting, I assume, womanly curves). Here are some men's Oakley sunglasses (same site):and some women's of the same brand:
Lastly, women's sunglasses are more likely to involve more color and patterning than neutrals (tan, black, grey). This is the first 9 for men's sunglasses:
(Admittedly the patterning on the second selection is hard to see at small sizes.)
In short, men's sunglasses generally reflect being more utilitarian whereas women's are, as with many things, also heavily conscious of being fashionable.