If you are on public transport, then you need to sit as if you have a suitcase or a huge bag between your legs. Legs spread wide apart not only look strange, but also interfere with those who are sitting next to you. This is simply uncivilized.
It is harmful to sit cross-legged - it interferes with normal blood flow. Also, do not slouch. If a woman is wearing a skirt above the knee, then she needs to monitor the position of her legs so that her underwear is not visible.