In sociology class we learn that Sex and Gender is not the same, however many people are uninformed about this. Sex refers to the natural or biological difference that distinguish male and females while gender refers to the state of being male or female (typically used with reference to social and cultural differences rather than biological ones). In the article created by Marina Watanabe, she link a video showing the difference between sex and gender. Sex is binary, male or female. While Gender is much harder to explain.
As you see from the image above, gender is important to understand, because you have to let the individual decide what he identify as. A boy might feel like he always felt like a girl and wants to become one. However in the video it explains that gender is a social expression and behavior. Society depicts how girls and boys should act, and this is why gender is important. Gender creates what a girl should be and what a boy should be. These norms that society creates, make people want to assume a new gender because they want to fit into a gender category. If you are still confused on this topic, check out this link.