Producing children is not a requirement but the potential to produce children has been a requirement for marriage in just about all cultures for a long long time.
For example, according to Wiki the law of Hammurabi is the first known written law on marriage. And one of the laws is that if a man marries a woman but does not have sex with her then they are not married.
"One of the oldest known and recorded marriage laws is discerned from Hammurabi's Code, enacted during the Mesopotamian world (widely considered as the cradle of civilization). The legal institution of marriage and its rules and ramifications have changed over time depending on the culture or demographic of the time.[20]"
http://en.wikipedia.org/wiki/Marriage
But hammurabi is not alone among people across history who saw the intimate
relation between marriage and procreation:
"According to Confucius, "Marriage is the union (of the representatives) of two different surnames, in friendship and in love,
in order to continue the posterity of the former sages, and to furnish those who shall preside at the sacrifices to heaven and earth, at those in the ancestral temple, and at those at the altars to the spirits of the land and grain."[5]"
anthropologists have proposed several competing definitions of marriage.[12] Edvard Westermarck, in his book The History of Human Marriage (1921) had said "The institution of marriage has probably developed out of a primeval habit. The relations between the sexes
and parental care among the Invertbrata"
"a union between a man and a woman
such that children born to the woman are the recognized legitimate offspring of both partners"[14]
"Marriage is a relationship established between a woman and one or more other persons,
which provides that a child born to the woman under circumstances not prohibited by the rules of the relationship, is accorded full birth-status rights common to normal members of his society or social stratum" [16]
Edmund Leach argued that no one definition of marriage applied to
all cultures. He offered a list of ten rights associated with marriage, including sexual monopoly and
rights with respect to children, with specific rights differing across cultures.[17]
"Various cultures have had their own theories on the origin of marriage. One example may lie in a man's
need for assurance as to paternity of his children. He might therefore be willing to pay a bride price or provide for a woman in exchange for exclusive sexual access.[21] "
IN Greece "A woman whose father dies
without male heirs can be forced to marry her nearest male relative—even if she has to divorce her husband first.[30]
"By 2009, all major English language dictionaries dropped gender specifications, or supplemented them with secondary definitions to include gender-neutral language or same-sex unions.[9][10][11]" Which means of court that before 2009 all major dictionaries included gender as an important part of the definition.
Because children are such an important part of marriage the "Nuer of Sudan allowing same sex marriages limited only to females who lack sons"
And what does that article say is the reason for marriage laws in the US?
A marriage bestows rights and obligations on the married parties, and sometimes on relatives as well, being the sole mechanism for the creation of affinal ties (in-laws). These may include:
* Giving a husband/wife or his/her family control over a spouse’s sexual services, labor, and property.
* Giving a husband/wife responsibility for a spouse’s debts.
* Giving a husband/wife visitation rights when his/her spouse is incarcerated or hospitalized.
* Giving a husband/wife control over his/her spouse’s affairs when the spouse is incapacitated.
* Establishing the second legal guardian of a parent’s child.
* Establishing a joint fund of property for the benefit of children.
* Establishing a relationship between the families of the spouses.