Just to add a little more to it. Males are favored because the first male in the family is expected to take care of their parents and any other dependants when they grow old. (Remember with whom Popo and An-Mei lived An-Mei's story? Her uncle and aunt.) Females are seen as more of a burden than an asset since they will need a dowry for their husband's family. The sooner you marry the daughters, the less the burden is on the family.

This has changed since Lindo's childhood. Many Chinese women that live in the large cities in China have adopted Western values about marriage, like marrying later in life and choosing their own spouse. In the rural villages, you may see the practice of arranged marriage still carried out but not as young as Lindo was when she married.

Female children are not valued in Chinese society, historically and presently. A father is ashamed if he has no sons to carry on his name. Therefore, a father would want to marry off his daughters as soon as possible to get rid of them. As wives, they need to give their husbands at least one son. That is their duty and responsibility in Chinese society.

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.