--Bona al la encycla lorica.
In Ethaya, it is custom that both men and women grow their hair long, and only marry when their hair is long. Women generally keep their hair spread out while men have them in ponytails. Upon marrying, they cut their hair (to roughly neckline), and they keep their hair at that shortness thereafter. They can divorce, after which they stop cutting their hair, (and they do this after they lose their spouse / become widowed), but then must generally wait until their hair grows long again before marrying. Those who become disgraced may either choose to cut their own hair (either by a certain amount or to the neckline for more serious issues, or even entirely shaved if very serious). Those who profess to be monks or nuns or otherwise never marry shave completely and keep it that way. Those who are punished for certain crimes such as adultery, may have their hair forcibly cut to shortness. All these are methods to make a person less marriageable.
Hence when Altair and Qura set out their hair continues to grow longer and longer (though due to various circumstances their hair winds up getting cut at several points along the way, particularly if the local customs disagree), then when they get married they both cut their hair. When Qura dies Altair vows that he won’t find another person to replace her, and so he continues to keep his hair short, decade after decade.