Earlier quoted context omitted.
You appear to be confusing grammar and biology. Why shy away from the word 'sex' when discussing sex?
Honestly? Because sex is a borderline bad word in polite company, and can get you into trouble if someone misses context and thinks you're talking about sex sex, while gender isn't a problematic word. The United States is so damned puritan in its culture that it's risky to use words that have a normal context that's problematic. [For instance, someone could overhear it out of context and think you're being inappropri…
If you're needing to use the word sex, it means you're talking about the difference in shape of a person's private parts. The word is exactly as socially acceptable as it seems to be.
If you're trying to compare a bunch of people in a room, like to say you should divide up teams by ___, you probably want gender, not sex. It's medical professionals who usually have cause to care about the sex of a person they know. Unless you're dating someone, and then it is sex sex you're talking about...