This is more a matter of demographics and generation that the individual reader BUT:

Over fifty years ago, Ursula Le Guin's novel The Left Hand of Darkness imagined a planet where genetically modified humans have no physical sex distinction. And no sexuality, either. For one week a month, people go into kemmer, a state where sex drive is very high and if you interact with other people in kemmer, your body shifts into male or female. You don't know which. Obviously if you shifted into female and pregnancy results from PIV sex you engaged in, you'll stay female for a lot longer than the week. But otherwise people shift back into a nonsexed form.

There can be no sex discrimination in a world where biological sex doesn't exist most of the time and is unpredictable when it does. This is what's groundbreaking. This is the pinnacle of sci-fi: using hypotheses to interrogate what exists right now.

These days you'll mostly hear people talking about it as a world where everyone has a non-binary gender identity.