If a lecture on sex can be conducive for underage then he/she can consent as well. How can you stop a girl in 16 marrying a man of age 60 if your law had declared her mature? Isnt it the case of superior marrying minor?
I would say teaching sex practices are counter productive as children would take it as permission granted or persuasion to him/her to have sex(protected or not protected) while being underage.
Without arguing, it can go both ways but I would put it as
Mon-martial Relationship:
There is no sense of responsibility so more abuse are expected.
Marriage:
Since couple aim at making the relationship last longer so less abuse and more care about each other.
In fact we observe in our society that more abuse are found in non-marital relationships. stats and data also suggest same.