rc,
You’re right that there are 19, 20, 21, 22, and 23 year old girls after the 23 year old boys. But why? Why didn’t the 20, 21, 22, and 23 year old girls all get married? The answer is, because there were more of them (per year) when they were 19 then there were 23 year old boys, because of population growth.
In other words, what you describe is true, but it’s not the cause, it’s the effect.