Actually, TTT, Merriam-Webster says that 'gender' is synonymous with 'sex'. The first definition is yours, but the second is 'sex'.
http://www.m-w.com/cgi-bin/dictionar...nder&x=15&y=15
The Queer Department at university, and in fact most of the people I know who are inclined to discuss such things, define 'sex' as the biological phenomenon, whereas 'gender' is an identity. As such, my sex is male and my gender is also male, but some individuals of the male sex (namely, transsexuals) may identify their gender as 'female'. Personally, I'm happy to use those definitions of the terms; they seem to be the common parlance at the moment, and bad things happen to those who cross the Queer Department.