They are not medically or legally different things.
I grew up in a medical family and I've worked in healthcare in various capacities for a couple of decades now. In any organization I've dealt with, "sex" or "biological" sex explicitly refers to your anatomy. That's important because biological males can't get cervical cancer and biological females can't get testicular cancer, for instance. They're the words used on the occasions when anatomy are relevant. Most medical organizations I've been around in the last decade or so distinguish between "sex" and "gender", which is what the patient presents themselves as. Sure, they're most commonly the same value, but they are separate database fields referring to different concepts.
But what you and other extreme liberals
LOL. You presume much, and wrongly. But con/lib aside, I've never encountered a single problem with referring to someone by their gender. The people who care to distinguish between sex and gender appreciate the respect, and understand when medical decisions require healthcare providers to discuss their sex instead. It's easy to be nice to people, so why not do so? It doesn't cost us anything.