Marriage is a human construct which has absolutely zero to do with biology. Marriage isn't natural at all, so nothing about biology applies to marriage, particularly not legal marriage.
"LIMITED" past tense. And the courts, the ultimate authority on that human construct we call the law, disagree with you who knows nothing about the law.
"Heterosexual couples are the only couples who can produce biological offspring of the couple."
Washington State Courts - Error
"We are dealing here with legislation which involves one of the basic civil rights of man. Marriage and procreation are fundamental to the very existence and survival of the race".
SKINNER v. STATE OF OKL. EX REL. WILLIAMSON | FindLaw
"The institution of marriage as a union man and woman, uniquely involving the procreation and rearing of children within a family, is as old as the book of Genesis...
Marriage and procreation are fundamental to the very existence and survival of the race." This historic institution manifestly is more deeply founded than the asserted contemporary concept of marriage and societal interests for which petitioners contend. The due process clause of the Fourteenth Amendment is not a charter for restructuring it by judicial legislation"
http://www.cas.umt.edu/phil/faculty/Walton/bakrvnel.htm
i]t is not surprising that the decision to marry has been placed on the same level of importance as decisions relating to procreation, childbirth, child rearing, and family relationships. . . .
It would make little sense to recognize a right of privacy with respect to other matters of family life and not with respect to the decision to enter the relationship that is the foundation of the family in our society.
ZABLOCKI v. REDHAIL | FindLaw
Under this standard, DOMA is constitutional because the legislature was entitled to believe that limiting marriage to opposite-sex couples furthers procreation, essential to survival of the human race, and furthers the well-being of children by encouraging families where children are reared in homes headed by the children’s biological parents. Allowing same-sex couples to marry does not, in the legislature’s view, further these purposes.....
Nearly all United States Supreme Court decisions declaring marriage to be a fundamental right expressly link marriage to fundamental rights of procreation, childbirth, abortion, and child-rearing....
But as Skinner, Loving, and Zablocki indicate, marriage is traditionally linked to procreation and survival of the human race. Heterosexual couples are the only couples who can produce biological offspring of the couple....
And the link between opposite-sex marriage and procreation is not defeated by the fact that the law allows opposite-sex marriage regardless of a couple’s willingness or ability to procreate. The facts that all opposite-sex couples do not have children and that single-sex couples raise children and have children with third party assistance or through adoption do not mean that limiting marriage to opposite-sex couples lacks a rational basis. Such over- or under-inclusiveness does not defeat finding a rational basis....
Washington State Courts - Error
Petitioners note that the state does not impose upon heterosexual married couples a condition that they have a proved capacity or declared willingness to procreate, posing a rhetorical demand that this court must read such condition into the statute if same-sex marriages are to be prohibited. Even assuming that such a condition would be neither unrealistic nor offensive under the Griswold rationale, the classification is no more than theoretically imperfect. We are reminded, however, that "abstract symmetry" is not demanded by the Fourteenth Amendment.
http://www.cas.umt.edu/phil/faculty/Walton/bakrvnel.htm