While having children is undoubtedly a person choice, anthropologically speaking, procreation is a perceived (and archaically assumed) feature of most relationships between the opposite sex. For those who seek to have children, and those that don’t, a monogamous relationship is usually deemed the most ideal unit from within which to procreate or simply to coexist with a partner. It represents stability, support and, bizarrely, social success of being in a relationship.
