June 12 marks the 50th anniversary of the Supreme Court’s 1967 ruling in Loving v. Virginia, which held that states could no longer prohibit marriages on racial grounds.

“Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State,” wrote Chief Justice Earl Warren. Like an earlier landmark decision on race, Brown v. Board of Education (1954), the Loving opinion was unanimous and brief—just 10 pages long. It was also unsurprising.

For...