不孝有三，無後為大 Bú xiào yǒu sān, wú hòu wéi dà / Bwu shiaw yeou san, wu how wei dah is frequently interpreted literally to mean “There are three unfilial things …”. Asking “What are the other two unfilial things?” is based on a misunderstanding of what Mencius was really saying.

Actually, there are many different acts that can be considered unfilial: insulting one’s parents, disobeying them, failing to provide for them when they get old, behaving wrongly thereby causing them to lose face, moving to a distant land and never visiting etc. etc.

The key to correctly understanding what Mencius said lies in interpreting the word 三 sān / san, which literally means “three” but is frequently used in a figurative sense to mean a vague unspecified number or “many”

三人行必有吾師 sān rén xíng bì yǒu wú shī / san ren shyng bih yeou wu shy “In any group of people, there will always be someone I can learn from”

三番五次 sān fān wǔ cì / san fan wuu tsyh “{literally} three times [and even] five times = many times”

Thus, a more accurate translation of 不孝有三，無後為大 Bú xiào yǒu sān, wú hòu wéi dà / Bwu shiaw yeou san, wu how wei dah would be “There are many unfilial acts, but the worst is not to bear a son” (to carry on the family line).

Of course, a literal translation of 無後 wú hòu / wu how would be “not to have a descendant”. In feudal times, raising daughters was often considered a waste (because they would eventually join their husband’s family), so only a boy would do. Fortunately, nowadays many families are just as happy to have a daughter.

As Quora User points out, whatever Mencius originally meant by 無後為大 wú hòu wéi dà / wu how wei dah is actually besides the point. What counts is how this statement has been traditionally interpreted: “this saying is much abused to force people to have male descendants”

This is similar to another phrase from Mencius: 齊人之福 Qí rén zhī fú / Chyi ren jy fwu “The blessing of the man from Qi”, which is now often used as a form of joking praise for a man who has relations with many other women besides his wife.

In Mencius, however, 齊人之福 refers to the story of a man who brags in front of his wife and concubine that he often enjoys meals with rich, important people. His wife and concubine eventually discover the shameful truth that he goes out to beg for leftovers from funerary offerings. Unaware that his wife and concubine know the truth, when he came home, the man continued to brag.

齊人之福 originally referred to a shameful situation, but in later times, its meaning was twisted into a form of praise.