Gender is not a synonym for sex. Gender refers to a cultural performance, with feminine typically associated with vagina and masculine typically associated with penis. A biological female in a dress has a female gender performance; a biological female who binds her breasts and purchases clothes from the men's section has a male gender performance. Gender performance is fluid and varies through time.
Just look at this article: Gender 'impacts on transplants'--it has nothing to do with gender, it is about biological men and women:
Women who get a replacement kidney from a male donor are more likely to reject the new organ, scientists suggest.This really kills me.