A young man has an illness that is hereditary. He doesn't have much to live, that or the illness will cripple him for life. There is a new breakthrough in medicine and science. One thing that needs to happen is his age is regressed to where it would be in the womb. This process was originally developed to help age those babies that were born too early, they age the babies to the point as if they had been developing in their mothers to the full nine months or old enough to be healthy. Then the genes that caused the illness are removed from his body. After the bad genes are removed the person would be brought back to the age they were before, unless they wish to remain younger than they were before. But that would cost a person extra money. While the main character was under to keep him from the pain, the doctors did something extra, either by accident or on purpose. Before he when in to save his life he was an okay looking guy with short hair and very little noticeable muscles. When he wakes up he finds not only is he feeling much healthier than he has ever felt in his life but his hair is much longer. They say that is the result of aging him so many years. One surprising thing is he is now a very attractive woman. His DNA if tested will register that he is 100 % woman as if he had been born that way. It is explained if he just went through a gender change he would have looked as plain as he had before but with just slightly more feminine features. But since they removed some genes they had to remove the entire genes from the original source. For example if the source of the illness had been his mother or his father and they tested his DNA as it was now it would be as if he wasn't related to that parent at all. Some of the stuff they deleted included the genes that included what made him look so average looking. But because they had to delete so much of his DNA and because one doctor watched to much science fiction movies and TV they added a little feline DNA. Now the main character has moments when she goes into heat. And has an uncontrollable urge to mate. Pregnancy may follow.
Another idea is that the main character stays a man but still becomes more attractive than he was before and still has an urge to mate. I'm thinking he mates with a person that had been his half-sibling. The half-sibling is related to him through the parent that the genes were deleted of. Now he no longer has any DNA ties to his former half-sister. They have sex and she gets pregnant and they get married.