The family unit is constantly evolving
Social attitudes to families are constantly shifting. Therefore, any definition is only valid at the time it was written.
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Because a family is a social construct, any definition can only be accurate for that time period. As social attitudes change so too does the sociological definition of a family.
No definition of a family can hold weight for eternity. The sociological definition of a family can only be accurate for as long as social attitudes remain fixed. As social attitudes towards marriage, sex, property, and economics change, the sociological definition of a family must also change.  In pre-industrial society, most families were a traditional nuclear family with one woman and man bonded through marriage, and their offspring. This social construct no longer applies to many post-industrial societies, forcing sociologists to revisit their definitions of a family.
Social and political attitudes change, but the core function of a family has remained the same since primitive family units roamed the Pangea supercontinent. Therefore, defining a family by what it does, rather than what it is, is still useful. A functionalist approach to defining a family can, therefore, effectively define a family.
[P1] Social attitudes towards the family unit are constantly shifting. [P2] Therefore, the sociological definition of a family is only valid at the time it was written.
Rejecting the premises
[Rejecting P3] A functional approach is not dependent on social attitudes.