Gender (Bender) Identity. Pat One

–noun

a person's inner sense of being male or female, usually developed during early childhood as a result of parental rearing practices and societal influences and strengthened during puberty by hormonal changes. (http://dictionary.reference.com/browse/gender+identity)

Often the terms sex and gender are considered to have the same meaning. This mistake is generally generated by the fact that most of the times sex and gender overlap and a person of a certain sex will identify in the corresponding gender but also by the need of society to label things. 

Who decided that this is a woman and this is a man?

It's just easier. Gender roles were to be a descriptive narration of reality and ended up as being undersized clothes we have to squeeze into if we want to be "normal". 

While lawyers and stocktraders were never bothered by the fact that cultural and social prescriptions were truly a matter of control rather than a matter of dictionary, artists (of all sorts) were struggling in a Quixotic fight against the windmills of the "it's always been like this and it always will". This intellectual form of rebellion is commonly referred to as Gender-Bender. Gender Bending is everything that goes against the traditional gender roles. When I go to the shop and buy a pair of jeans in the man's department I am gender-bending, I am doing it when I shave my hair or when I plan to be the breadwinner in the family. The gender roles are so over-generalized that almost anything we do could be considered a gender bending action.