What's the difference between gender and sex?

[TreeFlower]

So..... according to Merriam-Webster, sex is physical and gender is behavioral, cultural, or psychological?

Gender
2 a: sex <the feminine gender> b: the behavioral, cultural, or psychological traits typically associated with one sex
http://www.merriam-webster.com/dictionary/gender

Sex
1: either of the two major forms of individuals that occur in many species and that are distinguished respectively as female or male especially on the basis of their reproductive organs and structures
2: the sum of the structural, functional, and behavioral characteristics of organisms that are involved in reproduction marked by the union of gametes and that distinguish males and females
http://www.merriam-webster.com/dictionary/sex

[Constance]

Gender is an identity and sex is a pasttime.

NO! Wait!

Sex is part of one's physiology. For the most part, this is male and female. But, there is more to it than that as intersex persons do indeed exist, too.

Gender is a bit more complicated. It seems to me (at this point) to be partially physical and partially constructed. I'm not an androgyne because I chose to be. But, how I "present" my androgyne gender identity is something I choose. Therefore, to me at least, the presentation of gender is constructed.