I have yet to wear a skirt or dress, so I don't really have a need to keep my legs together. I know I'll never be one who crosses my legs, because that position is just
unbearably uncomfortable for me to hold for
any length of time. I like sitting with my ankles crossed. It just makes me feel girlier somehow. And sometimes I sit with my legs together and my knees folded and pulled up towards my face... Honestly, my sitting position probably changes constantly. I don't stick with any particular one.

It's funny... I really have no idea how most cis women sit when wearing skirts or dresses. All the cis girls I know are more of the casual blue jeans and T-shirts types.