Playing devils advocate here...children used to be married off and expected to have children, so when did it become wrong...and why.
Well you can´t really compare ancient times with modern times.
In ancient times boys and girls were considered adults at a lower age than tody.
I`d say around 13-15 years old they were already considered adults since they needed soldiers,hunters and so on while girls were considered women when they lost their virginity to those boys or when they got married and so on.
To my knowledge the more a human civilazation evolves the more childish a kid is since there is a lot more to learn now than those days.
Since we also have alot more information out there kids often need more guidence from adults at those ages even if biology says another.
The thing is...the mentality of young adults have been "babylised" from these 3 religions because at those times they thought that it was barbaric to young adults having sex with adults or whatever and they still do today.
I blame the modern world for it.
But yeah whatever...things are the way they are now...and will likely change in a few decades...