Introduction in first chapter
Usually the main purpose of a manga is explained in the first chapter, being the introduction to the complete story. In that sense both Natsu and Lucy would be the main protagonists. Both Natsu, Lucy (and Happy) are introduced in the first chapter. Natsu, who is searching for Igneel, while Lucy wants to join Fairytail and become a great Celestial mage.
Introduction of Natsu (and Happy)
Introduction of Lucy
Both Natsu and Lucy mention their goal in the story
On the other hand, the title usually points towards the main story too, like with One Piece, where Luffy is searching for the One Piece. In that sense Lucy should be more considered as being the main character as her story actually revolves around Fairytail, since she doesn't just want to be just any mage, she wants to be a
FairyTail mage. Natsu just happens to be in Fairytail, but his story doesn't really revolve around his guild. He just wants to find his father.
The Wikipedia page on Fairy Tail agrees on this, saying:
Fairy Tail follows the adventures of Lucy Heartfilia, a teenage wizard (魔導士 madōshi?),1 who joins the titular wizards' guild and teams up with fellow guild member Natsu Dragneel as he searches for the dragon Igneel.
Natsu is an important person to the story, and he is also introduced first of all, even before Lucy, but going by this Wikipedia quote I would say Lucy would be the main (and only?) protagonist.