I seem to be confused. Your definition of gender is something that is self-chosen, correct? maudlinstreet's argument appears to be that some people never make a conscious decision about their gender because they assume it's synonymous to their sex. However, they still have a gender (if pressed they tend to go with man/boy or woman/girl), they simply haven't made a choice about what it is.

Where does defining someone else's gender for them come into this?

Deleted the other comment because I was logged into the wrong account, sorry about that.