They not only traditionally dispose of most females for believing males are more valuable workers/warriors, but also of any offspring which doesn't look the right way after the first couple of months (if it doesn't have the right eye and skin color combination).
The people started as a normal civilization, was diminished by tribal and world wars and became purists/racists after the Great Humiliation, their last attempt of taking over the whole Continent. This forced them into a retreat deep into their traditional territory. Their numbers were down to about 10 000. No exact numbers are known because their Capital was destroyed during the Great Humiliation.
The Leaders blamed the defeat on the lack of purity of The People, who had, in earlier times, mixed with the others, so they forced the Pure Ideal on the rest of the population.
The People are ruled by a "Holy Oligarchy". The leaders call themselves Descendants/Heirs of the Powers (even if most of them know this is a lie and don't even believe the Powers exist) and take advantage of this to do whatever is best for themselves.
They do keep some females for reproduction, this makes females precious possessions and trading "coin". The ones with the right skin and eye color combination that most resemble their ideal of "purity" are the ones kept for the Leaders, the others are used as best serves their society.
Some, if they are a bit off, but are the firstborns of a mother, are kept to serve as slaves and possible reproducers, or to be given as rewards to the most loyal males. These females however will not even be allowed to see their newborn children, as lower casts are technically not allowed to reproduce.
Every male child is always allowed to live, as long as after about 8 weeks they show no over-obvious signs of having the wrong colour of eyes and skin. For instance, they might live if they have brown eyes instead of the desired black, or be freckly instead of having a clear overall colour. They might be a bit off the ideal and still be accepted in society but will never be allowed to breed.
A female who happens to be the firstborn of a mother is allowed to live, unless after the 8 weeks limit they present two unwanted traits. They may have freckles if their eyes are black, or have brown eyes if their skin is uniform. But if they happen to have both freckles and brown eyes, then they'll be "taken care of".
After that, from each and all newborn only the most "perfect" females are allowed to live.
The burden of perfection falls on the females, not the males.
How long could such a society survive before it crashed?