Maybe they are waiting for the right time. In the fanfiction, perhaps there are certain events or goals they need to achieve first. Like Natsu might be on a quest to become stronger to better protect Mirajane, and until he reaches that level, they keep their marriage a secret. Or Mirajane could be involved in some guild - related matters that require her full attention and she doesn't want any distractions that would come with a public marriage. Another aspect could be that they are just enjoying the secrecy and the special bond they have without the outside world prying into their relationship.
One reason could be that they want to avoid the chaos that would ensue among their friends and guildmates. Their relationship might be seen as unexpected, and they might fear the constant teasing and interference. For example, some of their more rambunctious friends could make a big fuss and it would disrupt their normal lives within the guild. Also, they could be worried about how their relationship might affect their standing in the guild as strong mages. They may want to keep their relationship private until they are sure it can withstand any potential problems that might come from being public.
It could be due to some external threats. There might be enemies who would target them if they knew about their marriage. For instance, if there are dark guilds or villains who have grudges against either Natsu or Mirajane, they could use their relationship against them. So, to protect each other and their new - found bond, they choose to keep it a secret. This also adds an element of mystery and danger to their relationship in the fanfiction.
Yes, there are likely fanfictions on this topic. The 'Fairy Tail' fandom is quite large and creative, so it's not uncommon for fans to write about such pairings getting married in secret. However, the availability might depend on the specific fanfiction platforms you search on.
Perhaps they just want to enjoy the early days of their marriage without the scrutiny of the outside world. They want to have a normal relationship for a while, just the two of them. They can learn about each other in a more private and intimate setting. It's like a honeymoon phase that they don't want to be interrupted by the media or public opinions.
It could be full of drama. Maybe they keep it a secret because of guild rules or some outside threat. For example, they might fear that a rival guild would use their relationship against them.
It could involve their daily life together. Maybe Natsu is still going on adventures, and Mirajane is supporting him while also taking care of their home. There could be cute moments like Natsu bringing back strange things from his quests and Mirajane just shaking her head with a smile.
Perhaps they leave because of a disagreement within the guild. Maybe there's a new policy or a decision made by the guild master that they don't agree with. For example, if the guild decides to get involved in a war that they think is unjust, they might choose to leave rather than participate against their beliefs.
They might face challenges from their respective guilds. Maybe Fairy Tail and Mermaid Heel have some sort of rivalry or rules that could make their marriage a big deal if it was known. So they have to be really careful.
Some Tumblr blogs are also dedicated to Fairy Tail fanfictions. Browse through the Fairy Tail tags on Tumblr and look for posts related to Natsu and Mirajane's relationship. There's a chance you'll stumble upon some great married fanfiction stories about them.
A sense of growth and change is often explored. Natsu and Mirajane might start to see each other in new lights. For example, Mirajane could help Natsu become more mature, and Natsu could inspire Mirajane to be more adventurous. Their married life becomes a journey of self - discovery and mutual support, which makes for an interesting and heartwarming theme in these fanfictions.
Well, their married life might be full of adventures. Natsu, being his energetic self, would probably take Yukino on wild dragon - slaying quests. They could discover new magical places together. And at home, Yukino would keep things organized while Natsu might accidentally break a few things in his exuberance. They'd also have to deal with their different guilds and friends' reactions to their secret marriage.
Some of their friends might be shocked at first. For example, Gray might do a double - take and be completely speechless for a while. But then he would probably be happy for them in the end. Lucy would likely be overjoyed and start asking a million questions about how it all happened and when. Happy would be flying around them in circles, squealing with excitement.