Just an FYI, you are cherry picking. Every other medical association and medical textbooks uses the classic definition of gender is the same as sex. As most of us are aware from filling out medical forums.

http://www.medilexicon.com/dictionary/36609

In all scientific fields it's important to have clear definitions. This is a topic that a lot of activist who are pushing a post modern world view are playing the game of changing definition. To the point where they can't even keep it straight themselves.

Who knows in the future there definition of gender might change. However, most people who are studying the topic use gender identify, sexual preference, biological sex, etc.