An idea for a story.

Or alternatively, an actual story told using not very many words.

....

In a future world where it is illegal to be "abnormal", all people born intersex are sent to an island to keep them away from all the amazing "normal" people who are far too busy having fun conforming to be distracted by anyone who doesn't conveniently fit into superficial "boxes".

As the years pass, a real sense of community emerges on the island. Those that are fertile in both aspects are highly valued in the intersex society, as they are considered the most useful to the collective survival of the island's people, compared to those intersex people who are deformed in a less useful way, for example having a vagina with a clitoris the size of a penis but no testicles, or having a normal penis and testicles but also fully formed breasts. Over time, due to the nature of natural selection, more people are born with the natural ability to both impregnate and also become impregnated.

Meanwhile, throughout the rest of the planet, a deadly virus spreads via an epidemic of bird diarrhea, killing the entire human race. However, due to some genetic quirk, all intersex people are somehow immune to this particular virus, leaving them as the only surviving humans on the planet.

After a while, the intersex people begin to realize that no crazy "normal" people have recently been to their island to threaten them and bash them over the head with hammers to prevent them from escaping the island (as previously used to happen on a random day at least once a week). So, feeling curious, the intersex people gradually begin to venture off the island, spreading across the planet on various rafts and rudimentary boats. They pass the time by getting each other pregnant, and also by playing Monopoly.

And so, a new single-gender human race develops. Same-gender marriage is obviously considered totally normal. Also, couples have the advantage of being able to both be pregnant at the same time, therefore making the raising of a family much easier and quicker (if both partners end up having twins, that's a large family created within the space of just 9 months + the shagging time).

In the new world, any freaks accidentally born with only one set of genitalia are labeled as "weirdos" and sent to live on a special island.

...

Note to Lee:
Maybe we could make this into a film? Hey, I could play the lead!