The idea of 'Naruto impregnates Kurenai' in fanfiction is rather odd. In the Naruto series, Naruto is more focused on his dreams of becoming Hokage and his relationships with characters like Sakura, Hinata, etc. Kurenai has her own story arc related to Asuma and the other ninja. Combining them in such a way goes against the natural flow of the characters' development and the overall narrative of the series. It might be an interesting, albeit very controversial, concept for a very small subset of the fanfiction community who like to explore extreme and non - traditional ideas.
It's a very strange and non - canonical idea. Naruto has his own set of relationships in the original story, and Kurenai is also part of different storylines. This concept seems to deviate a great deal from what we know about the characters and their relationships in the Naruto universe.
Well, from a creative perspective, it's an extremely out - there concept. Fanfiction is all about creativity, but this idea might be a bit too far - fetched. Naruto and Kurenai have different roles and relationships within the series. The Naruto series is mainly about ninja battles, friendship, and coming - of - age stories. This kind of fanfiction concept doesn't really fit into those main themes. It would be a very different and perhaps not so well - received take on the Naruto world.