In short, so that they may produce heirs as quickly as possible. Male children were a source of pride and evidence that the gods favored you.
For Lindo to go to such extremes to avoid sleeping with her husband and thus avoiding creating these heirs was an unimaginable breech. The absolutely unthinkable act is part of the reason, along with her craftiness, that Lindo is able to pull of her ruse. The fact that she manages to have sons with her chosen husband is evidence to Lindo that she has not been condemned. The bracelets that she wears, given to her to commemorate the birth of her sons, helps her to always remember.