According to the Fallout Wikia, 22 outfits are restricted to male dwellers only. Only 6 outfits are exclusively female.

This is incredibly imbalanced and frustrating.

My dweller population is about 50/50, but my female dwellers can't get the same wardrobe bonuses as the male dwellers sharing the same assignments.

All dwellers should have access to the same number of outfits. Ideally, ALL outfits should be wearable by ALL dwellers. If you must restrict clothes by gender, you have a 16-outfit deficit (making more male-only outfits unisix and/or adding more female-only outfits).