I doubt there are legitimate 'reasons' for such inappropriate concepts in proper fictional works. However, in some very bad - taste or non - mainstream works, it might be an attempt to shock or gain a certain kind of attention, but this is not acceptable in the realm of good storytelling.
If we assume there are works out there with such a concept (which is highly unethical), it could potentially be a misrepresentation of human relationships and sexuality. Some creators might be trying to play on baser instincts in a very wrong way, without considering the negative impact on the audience. This is not something that should be encouraged or supported in the world of fiction.
I'm not aware of any such stories being popular in mainstream literature as this involves vulgar and inappropriate content that goes against the norms of proper literary works.
One possible reason could be revenge. If the guy has played a mean prank on the girl before, she might want to get back at him in a similar, albeit more extreme, way. For instance, if he poured water on her during a party, she might plan to strip him as payback.
Sometimes it's about power. Girls might feel that by bullying a guy, they can show their dominance. In a group setting, if a girl wants to be the leader or in control, she might pick on a guy to assert her authority. It's not right, but it happens. Another factor could be insecurity. If a girl is insecure about herself, she may take it out on a guy by bullying him.
One possible reason could be curiosity about human sexuality. Erotic stories can be a form of exploring sexual feelings and relationships in a fictional and somewhat safe way.
The non - existence of 'birth erotic fiction' can be attributed to multiple factors. Morally, it crosses a line as birth is a pure and life - giving process, and eroticism is about sexual pleasure and desire. They are fundamentally different in nature. Aesthetically, it's not appealing. The beauty and significance of birth are in its life - creating aspect, not something to be sexualized. Culturally, most cultures view birth as a solemn and family - centered event. Erotic fiction, on the other hand, caters to a different set of interests mainly related to adult sexual relationships. There's no market demand for such a strange combination either, as readers of erotic fiction are looking for different types of stories that are more in line with the typical themes of the genre.
Maybe some people just want to create shock value. They think by coming up with such a radical and out - of - place concept, they can get more attention.
To make a statement about gender equality. By feminizing a guy, they are showing that there are no strict boundaries between genders and that anyone can experience the other gender's typical things. It can be a way to break down gender stereotypes. For instance, if a guy is made to do 'girly' things like knitting or wearing makeup, it shows that these activities are not exclusive to girls.
One reason could be the exploration of taboos. Some writers might want to push the boundaries of what is considered acceptable in society. Another could be to shock the audience and gain attention. However, these are not positive or ethical reasons.
Well, one reason could be that in the world of fanfiction, people like to push boundaries. Writers might be interested in exploring the contrast between the concept of love (lesbian relationship) and extreme violence (cannibalism). Also, for some, it may be a form of expressing their darker, more unfiltered creativity. However, this is still a very niche and often controversial area. It might also attract a small group of people who have a particular fascination with such extreme combinations of themes.
Older guys may also possess more life experience. They can offer different perspectives and advice, which is fascinating for younger girls. In the story, perhaps the girl was drawn to the older guy's stories about his travels and adventures around the world.