If you're an older man, ask the next 15-year-old child on the street you see if she'd like to "marry" you. You'd be looking through bars when the authorities found you.
Let's put the shoe on the other foot, now, and see if you still scratch your head. Let's father some boys with docile, brainwashed, ignorant sheep-like men for fathers, raise them in a remote fenced compound ruled by domineering women, and when they are 13, "marry" them to the old women in power for "spiritual purposes," denying them free will in any matter. Actually, they'd get off lucky, because they wouldn't be pregnant and saddled with the old perv's child.
"Marriage" with parental permission is a farce with these cults. The "bride's" father will grant the permission, because it ultimately benefits a society that panders to his own perverted sexual nature (I suspect money or power changes hands, as well), and the mother of the "bride" grants her permission by the enforced absence of her own free will, knowledge and judgement.
Those old pervs are pretty smart. They know if they can get these poor child "brides" pregnant, they will stay with the cult to be with the babies they are forced to bear, who will then be prepared to be the next crop of "brides."
"Brides"; sheesh.