You can say that nothing is mentioned as such that this is reserved for woman. This show that this woman is taking your advantage by saying that she is a woman and need a place to sit. In this situation, you can tell this to bus conductor about her misbehaviour. He might explain her that this is their bus and  there is no rule like that. This woman is taking advantage of her being a woman and showing  pity onto her. If than also she doesn’t understand than you can report her case to the travelling agency so that they can take any action.