Adoption: Right or Duty?

As long as this question remains unanswered, the purpose of adoption institution will also not be unfolded. The question is that, whether adoption is the right, or the duty of the society?
Paragraph 2 article 5 of Convention on the Elimination of all Forms of Discrimination against Women answers this ambiguity: 

To ensure that family education includes a proper understanding of maternity as a social function and the recognition of the common responsibility of men and women in the upbringing and development of their children, it being understood that the interest of the children is the primordial consideration in all cases.

Key points: the role of maternity is a social function. Up-bringing of the child is a co-responsibility of man and woman (and not necessarily father and mother). The benefits of the child (and not only his/her rights) have priority in all cases.  
 Article 21 of the Convention on the Rights of the Child states the following on adoption: 

States Parties that recognize and/or permit the system of adoption shall ensure that the best interests of the child shall be the paramount consideration... 


Key point: priority of children's best interests, in line with implementing this institution.
The highly important point in these conventions is that, the term "interests" was used instead of "rights" to children's advantage. But what's the difference between interests, and rights?
I address this difference by providing an instance. According to the universal declaration of Human Rights, having a standard life is your right, but if the status of your life is better than that standard, then that becomes your interest.
When it's stated that child's interests have priority in all cases, then that means, if the court has to choose between one of the two options, and one option is society's rights while the other is child's interests, then the chosen option, has to be the child's interests.
It's clear that the purpose is to defend the interests of the children, thus just as the role of maternity is recognized as a social function to protect the interests of the child; adoption is also considered a social function.
It may be claimed that adoption is both a duty and a right just as the role of maternity is both a duty and a right.
Paragraph 1 article 9 of the Convention on the Rights of the Child states the following: 

States Parties shall ensure that a child shall not be separated from his or her parents against their will, except when competent authorities subject to judicial review determine, in accordance with applicable law and procedures, that such separation is necessary for the best interests of the child. Such determination may be necessary in a particular case such as one involving abuse or neglect of the child by the parents, or one where the parents are living separately and a decision must be made as to the child's place of residence.

Key points: the interests of the child precede the parental rights of the parents. The guarantee from the membered states is that, the children will not be separated from their parents, despite their own will.
There are two cases in which the child is separated from his/her family: Juridical compulsion for protecting his/her rights and the other one is personal will. Thus, the role of maternity is a social function and the principal right is under the dominance of the two individuals: the child and the court.
So, the institution of adoption is a social function for protecting the interests of the children.
Adoption can be called substitution of the child. That means, a child that has no parents and is socially (and not naturally) no body's child, takes the place of a substituted child by being adopted into a family. And his/her keepers will become substituted parents and will have duties same as the duties of the real parents. 
Is a gay couple, eligible to have the capacity of adoption?
-No
Why?
Paragraph 2 article 5 of the Convention on the Elimination of all Forms of Discrimination against Women has declared the up-bringing of the child, the co-responsibility of woman and man. The key point is that the responsibility of the child's upbringing should be given to a family consisting of a man and a woman, when the child has no parents, or doesn't have a father or a mother.
Also, since the child suffers from the lack of parents and since the purpose of the institution of adoption is to protect the interests of the child and to fill the gaps in his/her life, so the family that adopts the child should be the gap-filler of the lack of parents. That means they should provide the affection and spiritual needs of the child. Otherwise, the orphanages can shelter and protect the children.
It should be noted that adoption, is actually the substitution of the child. This title of child's substitution is achieved, when that family is naturally able to breed. Is a group (or a family) consisting of two women or two men, naturally able to breed?! 
Is a child that is adopted into such a group able to take the place of a substituted child?!