Assume: in ancient Arabia, life was very harsh; tribes survived by raiding each other; 100% of adult men would go on a raid; women do not fight; fighting was brutal and merciless; casualties were high; no male prisoners were taken; wounded were killed; then ...
The first raid is inconclusive and 20% of men die on both sides. Now there are 100:80 women to men.
The second raid is also inconclusive, with the same casualties. The ratio is now 100:64.
The third raid is a rout, a total victory. The victors again lose 20%, but all the losing tribe's men are killed, and all their women are taken.
The victors now have more territory, more camels, more date palms and more wells.
The female to male ratio is 100+100:51
Of course, you can reduce the casualty rate and increase the number of raids, but something like that could be quite close to historical reality.