What really overturned "traditional" marriage (Hint: it wasn't us)

Marriage Scholar Stephanie Coontz

The redefinition of traditional marriage began about 250 years ago, when Westerners began to allow young people to choose their partners on the basis of love rather than having their marriages arranged to suit the interests of their parents. Then, just 100 years ago, courts and public opinion began to extend that right even to marriages that parents and society disapproved.

...In 1967 the U.S. Supreme Court ruled it unconstitutional for states to prohibit interracial marriage. In 1987 it upheld the right of prison inmates to marry.

...The longstanding idea that the validity of a marriage depended on the ability and willingness of a couple to have children was eroded on two fronts when married couples who were not biologically capable of having children won access to other ways of starting a family—through artificial insemination, sperm donors, surrogate mothers, and liberalized adoption laws—and married couples who did not want children won the right to use contraception. ... 

But the most important cultural change that has increased support for same-sex marriage is the equality revolution within heterosexual marriage. ... 

Between the 1970s and 1990s... most Americans came to view marriage as a relationship between two individuals who were free to organize their partnership on the basis of personal inclination rather than preassigned gender roles. Legal codes were rewritten to be gender neutral, and men’s and women’s activities both at home and work began to converge.
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The growing acceptance of same-sex marriage is the result of these profound changes in heterosexual marriage. It’s not just the president’s views on marriage that have evolved. Marriage itself has evolved in ways that make it harder to justify excluding same-sex couples from its benefits and obligations.