Eileen had gotten so caught up in the pleasure of fucking her girlfriend she hadn't noticed what she did when their pussies rubbed together. Her focus had been on stroking her clit against Ty's, and the thick white excess semen had soaked into her pussy. The consequences of having a man's juices inside her woman's cavity were quietly affecting her.
She had subconsciously just assumed his ejaculate would die off along with the rest of her male self- but since Eileen came only a couple of minutes before the two of them had sex as girls, the cum hadn't dried out and killed the millions of sperm it carried.
Naturally the reproductive cells weren't aware the vagina they swam inside had previously been the testicles that spawned them- from their perspective, this was a lucky break of insane proportions. They had been near death, but would now get to accomplish their instinctual goal!
They swam through Eileen's freshly feminized body eagerly, soon reaching her womb. Compared to the torrents he had pumped into Ty their numbers were sparse, but as everyone knows, it only takes a single sperm to fertilize an egg.
This wouldn't be a problem if Eileen hadn't been fertile, but the inrush of feminizing hormones had caused her to ovulate, just as it had done to Ty when she changed sex. The egg that had emerged inside her naturally ached to get fertilized.
So it was that the reproductive cells intertwined. Her egg was surrounded by desperate, unyielding seed which bore away at it. They saw their opposite number and worked together, not knowing they came from the same person.
The last time this had happened in Eileen's body, she had paradoxically been older than she was now. But in any case, the egg was fertilized and lodged into her womb. The young woman was pregnant, and she was both the father and mother.