Eileen blushed too, but she had to admit, the idea did kinda turn her on- it had been surprisingly nice to be regressed to the foundations of life and then aged to the point of fertility, and the idea of switching sexes again was naturally very appealing. Not to mention the idea of an infinite loop of sex. "It sounds fun," she smiled. "Let's do it."
Once she said that, Ty smiled back and eagerly slid his shaft between her pussy lips. He gasped at the sensation of getting to fuck a woman, only now realising this was his first time, but holding back his intense hormonal instincts while he waited for Eileen to say he could move.
"Ty," Eileen breathed, her voice husky and breathless at the sensation of being penetrated, which felt almost unfamiliar after the time she'd spent as a man. "Fuck me already!"
Naturally, Ty's hips pounded into her instinctively and speedily, and his head brushed against her G-spot tantalisingly to drive her closer and closer to her orgasm with every thrust.
They both peaked in perfect sync, and as Ty released his seed inside Eileen's awaiting pussy, the two of them left their old bodies and found themselves inside sperm that found and claimed Eileen's eggs, each of them fertilizing one and making the woman's body pregnant.
As their bodies developed, they each noticed the contrast between how they had grown the first time and how they grew this time once they reached the point at which their bodies' sexes manifested. Eileen had previously come from a sperm with an X chromosome, but this time came from one with a Y- the intervention of the signal that made her develop masculine traits felt like a secret she had never known being told to her, knowledge of how to be male, and he formed a penis and testicles, gained the capacity to fertilise others with his seed.
By contrast, Ty had come from a sperm with a Y chromosome before, but now came from one with an X- this time the absence of that signal that had made him develop masculine traits was like having distractions tuned out, his body able to focus properly and be female, and she developed mammaries, a vagina and ovaries, gained the ability to be impregnated and bear young.
Soon they aged to the point where they were to be born, and were birthed from Eileen's vagina, at which point Ty wished away their old bodies and they developed into adulthood.
They looked at each other in arousal and kissed passionately. "Wanna switch again now?" Ty asked in her higher-pitched girl's voice.