Yes, there could be fanfics with this theme that are written in a tasteful and appropriate way. For example, some might handle it in a very platonic or alternate universe kind of scenario where the concept is more about the relationship growth and the idea of family in a non - explicit manner.
A Natsu x Lucy pregnant fanfic could be full of heartwarming moments. Think about Natsu being overprotective of Lucy during her pregnancy. He'd probably be a bit clumsy at first, but his love for her would shine through. And Lucy would be the one trying to keep things in order while also dealing with her hormonal changes. It could also add a new dynamic to the Fairy Tail group as a whole.
One plot idea could be that a magic spell goes wrong and Lucy ends up pregnant. Natsu is initially shocked but then quickly embraces the idea of being a father. They then have to deal with the guild's reactions, some of which are positive and some negative.
There are a lot of fanfics out there, but ones with sexual content are not in line with the general positive spirit of the Fairy Tail franchise. We should look for fanfics that are more about the growth of the characters, their relationships in a non - sexual way, like how Lucy and Natsu support each other during battles and in their quest for magic.
There could be such a fanfic out there. However, the world of fanfiction is vast and unregulated. It's possible that it exists on some niche fanfic platforms. But it's also important to note that some fanfics may deviate from the original source material in ways that some fans might not like. So, even if it exists, it might not be to everyone's taste.
Family values would be a significant theme. The story could focus on how their friends and the extended family react to the news, and how they all come together to support Natsu and Lucy. Also, the theme of growth - both personal and as a couple - would be present. Lucy might grow in her confidence as a future mother, and Natsu could grow in his responsibility.
Well, one possible plot could be that Natsu and Lucy's relationship evolves from friendship to something more romantic. Then, an unexpected pregnancy occurs which throws their lives into chaos. They might have to face the reactions of their friends and deal with their own fears and insecurities about becoming parents.
Their friends would likely be shocked at first. Happy for them, but also a bit worried. Erza, for example, would probably be very protective. She might start giving Lucy all sorts of advice on staying healthy and safe during the pregnancy. Gray might be a bit awkward around the whole situation at first, but would eventually warm up and offer his support in his own way, like helping Natsu with any odd jobs or tasks so he can focus on Lucy.
Writing fanfic with sexual content is generally not appropriate, especially when it involves characters from a mainstream series. It goes against the family - friendly nature of many of these works and can also be disrespectful to the original creators and the fan community at large.