Well, the meanings have slipped so far that they are now used interchangeably, but they're not. It's all about destroying the language so people can't think clearly about certain topics (see George Orwell, Politics and the English Language). Sex is a concept that applies to biological organisms. It is definite and fixed. Gender is a concept that applies to  grammar. It is arbitrary and fluid. Using the latter helps destroy the meaning of the former -- cultural Marxism at work.