Life is much more complex in today's world, requiring more years of education and preparation for adult life. In our society early teenagers are still children. As such, they are minors and are unable to give informed consent to a marriage covenant. Therefore, Biblical covenant marriage to such a minor is not possible in our society. It is abuse, because it is an adult taking sexual advantage of a child.
Society rightly seeks to protect children from such abuse.
Another abusive practice of some fundamentalist Mormons involves incest of various kinds. Among the worst is the practice of marriage to both a mother and her daughter or granddaughter. That is wickedness. A marriage to a mother and daughter distorts the relationship between them, undermining the mother's authority over her daughter. If the father will not restrain himself from this evil, and the mother does not step in to prevent it, then it is up to society to step in on behalf of innocent victims of this wicked behavior.