Consider the 14th Amendment, Section 1:
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
The 14th Amendment followed the 13th Amendment (1865) that prohibited the inhumanity of chattel slavery. The mere prohibition of slavery was insufficient, and the 14th Amendment established de jure equality of all men for the pursuit of life, liberty, and property. Also, African-American male suffrage needed the 15th Amendment (1870).
These marvelous amendments, however, initially failed to prohibit unjust discrimination in the form of racial segregation. For example, the US Supreme Court Plessy v. Ferguson (1896) defended de jure racial segregation based on the pretense of "separate but equal." Also, the 15th Amendment never established suffrage for all adults regardless of gender.
De jure corrections of the above unjust inequalities included the 19th Amendment (1920) that established women's suffrage; the US Supreme Court Brown v. Board of Education (1954) that decided public school segregation was unconstitutional; the Civil Rights Act of 1964 that banned discrimination based on race, color, religion, sex, or national origin in employment practices and public accommodations; the Voting Rights Act of 1965 that suspended poll taxes, literacy tests, and other subjective voter registration tests; and the Civil Rights Act of 1968 that legislated equal housing opportunities regardless of race, creed, or national origin.
Landmark US court cases re marriage equality focused on the 1996 Defense of Marriage Act (DOMA) and the 2008 California Proposition 8. DOMA was a federal law that allowed states to refuse the recognition of same-sex marriages granted by the laws of other states. Proposition 8 became a California state constitutional amendment that banned same-sex marriage.
Chief Judge Vaughn Walker of the United States District Court for the Northern District of California ruled in 2010 that Proposition 8 was unconstitutional while citing the 5th Amendment and the 14th Amendment Due Process clauses and also the 14th Amendment Equal Protection Clause. He concluded that California lacked a rational basis for denying lesbians and gays from the legal benefits of same-sex marriage. For example, all opposition to California same-sex marriage involved nothing more than traditional approval of opposite-sex marriage and traditional disapproval of homosexuality. Three years later in 2013, the US Supreme Court made a unanimous dismissal to the last appeal from Proposition 8 proponents. The US Supreme Court in 2013 also struck down DOMA, Section 3.
The US Supreme Court trend that favors marriage equality suggests the end of de jure marriage inequality based on the implications of the Due Process Clause and the Equal Protection Clause. This predicted end of de jure discrimination, however, will have no effect on any given ecclesiastical definition of marriage and annulment, which has always been separate from state and federal laws. This issue is de jure equal opportunity for LGBT Americans who yearn for marriage. 2015 is the year of US marriage equality.