The exclusive preference for prepubescent girls for marriage is not forbidden in Islam. Islamic jurists have categorized wives as either “Kabira” (adult) or “Saghira” (prepubescent). A Muslim can only have sexual acts with his prepubescent wife within bounds of marriage. Prophet Muhammad p.b.u.h married Hazrat Aisha when she was six but did sexual intercourse with her when she was nine (it is unknown whether she had menstruated or not at the time of consummation of marriage). On the basis of this jurists have opined that marriage should be only consummated with wife if she is nine or older. For sexual acts with wife under the age of nine, the husband has to seek permission from his father-in-law.
