Natural Family
Natural Family is one produced by the committed "cleaving" marriage of a man and a woman. This couple brings biological children into such a family through conception and birth. Other children can be adopted into such a family.
Productive Marriage is the Foundation of the Natural Family
This is a marriage between a man and a woman. This gives the ability of this couple to produce children that have biological ties to the parenting couple.
Children are Additions to the Natural Family
Most people view a family as any sort of conglomeration of people that care for children and that the presence of such children create a "family".
The Natural Family is centered on the marriage of a productive couple, which is a couple who can conceive children amongst themselves. The couple forms a family and children are brought into this family.
Adoptive Families
Couples can choose to bring non-biological children into their family as well.
To bring other children into a NaturalFamily with biological children is the ideal situation. Otherwise "barren" Natural Family couples that adopt children without their own biological children are less ideal yet noble, as vital biological ties among family members are absent.
Other family constructs that may desire adoption can be working with situations and attitudes that are detrimental to proper child development and I personally discourage such arrangements.