From the Wiki page:

Frustrated, the Smurfs got back at her by making her think that she had put on weight and was becoming fat, thus causing her to fall into a state of depression. Papa Smurf took pity on her and took her to his laboratory, where they locked themselves in for several days before emerging. Smurfette now had long, flowing blond hair and an appealing character. This caused every Smurf of the village to fall in love with her.

Eeeeeeeeiiiii…. incest!!!