Initiation
Tribes often have initiations. In many African tribes, initiation means circumcision of males and sometimes circumcision/genital mutilation of females.  Initiation is thought to be necessary for the individual to be regarded as a full member of the tribe. Otherwise, the individual may not be allowed to take part in ceremonies or even in social ritual such as marriage. A man will not be allowed to marry or have any special relationship with a woman who didn't go to an initiation, because she is not considered as a woman.