Something Liam and Alex had forgotten in their horniness was that just because everyone had changed sex didn't mean their transformed bodies weren't fertile. If you had sex by having a penis fuck a vagina and the penis came inside, it would very likely lead to pregnancy. And since Liam had just fucked Alex raw, their bodies were preparing to get her pregnant.
Alex's fertile womb was flooded with Liam's thick semen, and his sperm had begun to swim through Alex's passages, sensing that deep inside the young fertile woman was their opposite number, something which would ensure they developed into offspring. Soon enough, they reached it- an egg that was nestled in Alex's ovary awaiting fertilization. The energetic seed set about wearing away at its outsides from every angle, and it soon became inevitable it would capitulate.
Soon one of them reached inside the egg and claimed it with its gametes, and the egg began to move down into Alex's womb, settling into place and making the woman who only about an hour before had been a man pregnant with the child of a man who had until then been a woman.