Not sure if this qualifies, but Orlando immediately springs to mind, as does Jeanette Winterson's Passion and The Power Book.
Also not sure if this qualifies (that's going to be a recurring theme...) but what about Iain Banks's The Wasp Factory? That's at least approaching the sci-fi/fantasy boundary...
ETA: And I'm guessing that some manga would feature this, some of which would be fantasy/sci-fi?
Oh! *palmface* I can't believe I didn't remember this. The Breeds of Man by FM Busby is set in a world that managed to produce a vaccine for HIV... and, a few years after it has been distributed to everyone, they discover the side-effect: sterility. So then the scientists work to solve that, and end up producing babies... that change sex depending on whether or not they're near someone's ovulation cycle. It's a bit pulpy, but that's why I love it (it's an 80s trashy sci-fi novel), and most of the kids, having been assumed to be boys or girls and then experiencing a change mid-way, spend the rest of the time pretending to be boys (or girls), in order not to let their secret escape.
here via monanotlisa
Also not sure if this qualifies (that's going to be a recurring theme...) but what about Iain Banks's The Wasp Factory? That's at least approaching the sci-fi/fantasy boundary...
ETA: And I'm guessing that some manga would feature this, some of which would be fantasy/sci-fi?
I found this likely list.
Oh! *palmface* I can't believe I didn't remember this. The Breeds of Man by FM Busby is set in a world that managed to produce a vaccine for HIV... and, a few years after it has been distributed to everyone, they discover the side-effect: sterility. So then the scientists work to solve that, and end up producing babies... that change sex depending on whether or not they're near someone's ovulation cycle. It's a bit pulpy, but that's why I love it (it's an 80s trashy sci-fi novel), and most of the kids, having been assumed to be boys or girls and then experiencing a change mid-way, spend the rest of the time pretending to be boys (or girls), in order not to let their secret escape.
Plus there's explosions.