Families of orientation vs families of procreation
There is an important difference between the family you are born into, and the family you create with a lover.
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There are two types of families and each one has its own sociological definition.
A family of orientation is the family that a child is born into. This may be characterized by blood relations. A family or procreation is one that is formed through marriage. These distinctions are important when defining the sociological constructs of a family. Each family has its own criteria and must be considered as different sociological groupings. 
While the distinction between families of orientation and procreation makes sense on an individual level, it does not make sense on a macro level. Beyond the individual, both families likely function the same way. They probably interact with society in the same way, and have many similar characteristics. Therefore, a sociological definition can be developed that is inclusive of both types of family.
[P1] The family you are born into has a different dynamic to the one you enter into when you procreate. [P2] Therefore, one sociological definition of a family cannot apply to both. [P3] Therefore, the sociological definition of a family depends on the type of family under consideration.
Rejecting the premises
[Rejecting P1] The two types of family are different from the perspective of the individual, but not in the way they function or interact with wider society.