Before the times you speak of ("the beginning of our evolution"), it was more a matter of necessity... You probably would be dead by 25, so better get going at 15! If one looks back to say, oh, the 1910s, sexual intercourse was not a common thing for civilized 14 year olds to take part in. I'm not sure where in the world this has been widespread and accepted in recent times.
I see your point about the young marriages... However, is today's youth as mature as say, a young English lady who was in an arranged marriage 250 years ago? More likely than not, that girl was not the same as a 14 year old living in modern Boston.
It's just that I'm not too optimistic about the decisions youth are making. I'm only 18, so I'm in the same generation as a one who is 14 or 15. Nonetheless, I am very rarely impressed with most of the younger teens I meet. Hell, it's difficult to find someone *my* age who can have genuine conversation.
It's "sad" to me because there are so many other things someone this age should have on their minds. Sex is a big deal, despite what you think, and it's not a worry that Mary Jane in the 8th grade needs to have clouding up her thoughts.