My understanding is that sex and gender aren't as clear cut as people used to think so a growing number of people use sex and gender interchangeably by referring to it as "sex/gender" or "gender" as a catch all term. Think about it that way: Sex is physical, like organs, hormones, height, muscle mass, ... and gender is social like behavior, norms, expectations. So telling boys to eat up to they get tall and strong and letting them do a lot of sports while telling girls not to get fat and simultaneously to do more creative stuff that isn't too active is gender. But when boys in fact grow tall and muscular, that's testosterone and therefore sex and has nothing to do with gender.

The argument is that gender and sex are too multifaceted and too intertwined to meaningful separate them. That's at least one take away a had from reading "Who's Afraid Of Gender" by Judith Butler. It's not too long and an easy read. Highly recommend.