Opponents to marriage equality in the United States may refer to religious reasons or to beliefs about the supposedly eternal nature of the family. Answering the question posed begins with providing definitions of “Western” and “family.”
The idea of the “Western” presumes a clear distinction from the “East,” which is not supposed by empirical research. Within the diverse societies often considered to be part of this imaginary West, primarily Europe and its former colonies in North America, there have always been a number of different family forms. These forms have changed considerably over time.
One good example is the current prohibition against polygamy in the United States. Such laws were enacted in numerous states after polygamy became a more widespread practice. While it is associated with the Church of the Latter Day Saints, the practice of having multiple spouses at one time was common among a number of different religious groups. Such legal changes prompted some families to move to Mexico to avoid prosecution.
Another example is the overall trend from extended family co-residence to nuclear family. Only in the last few generations has it become common in the United States for grandparents to live separately from their adult children and grandchildren. The idea of a separate residence of just parents and children did not become the norm until the mid-20th century.
Adoptive families are another kind of family that has changed over the years. It was extremely difficult for single people to adopt children, despite the large number of children who were in orphanages or foster care. One of the reasons often used to support ending marriage discrimination against same sex couples is facilitating a married partner’s adoption of their spouse’s child or serving in such capacity as making medical decisions for them.