Define sex and gender. Is anatomy destiny? Why or why not? ?

Both of those terms have numerous definitions, all of which can be found in dictionaries. As for anatomy being "destiny," it's not clear what you mean by that, so in the vaguest of terms, all I can say is that human anatomy is predetermined by genetic coding, but it is not immutable as it is altered by injury, illness, medicines, surgery, and so forth, as is DNA itself by injury (how cancer happens, for example), illness (retroviruses, for example), childbirth (fathers' DNA becoming incorporated in mothers' DNA), radiation, chemicals, surgery, blood transfusions, and various other technologies. 

Sex refers to characteristics of the body, gender is a grammatical construct that relates to pronouns like "he" or "she"

Sex: your body. Biology.

Gender: the stereotypes society associates with your sex.

Obviously you can't change your sexed body into the other sex. It's impossible. No matter how many hormones you take or plastic surgery you get, every cell in your body is still an XY or and XX. None of this has anything to do with your _personality_ which seems to be what the dimwitted Millennials think is their "gender." Any sex can have any kind of personality.