'Lost sex fanfiction' is not a common or well - defined term in mainstream fanfiction circles. It might be a very niche or even misnamed concept. It could be that someone is trying to describe a fan - written story about lost relationships where there are elements of a romantic or sexual nature involved, but again, this is just speculation without more details.
Well, that's not entirely true. There are a wide variety of manhwa with different themes and subjects. It might just seem that way because the ones about sex get more attention or publicity.
Since it's a fanfic related to 'The Lost Heir', it might be a creative take on the original story. It could involve the heir character in various scenarios that the official story didn't touch upon. For example, it might explore how the heir would react in different political situations within the fictional world. Or it could be about the heir's relationships with other characters, like forming unexpected alliances or facing new enemies, all while adhering to the general setting and themes of 'The Lost Heir'.
This term suggests a sexually explicit fan-made manhwa centered on a guy like you. It might be found in certain online platforms or communities that focus on such adult-oriented fan creations. However, accessing or engaging with such content should be done with caution as it may not be suitable for all audiences.