When Donald J. Trump was elected president, gay and lesbian leaders warned that their far-reaching victories under Barack Obama — including the Supreme Court’s decision legalizing same-sex marriage in 2015 — were in peril, endangered by the imminent arrival of scores of conservative judges and full Republican control of the federal government.

So it would be an understatement to say that gay rights leaders and supporters were surprised by the court’s ruling on Monday that the 1964 Civil Rights Act protects gay and transgender Americans from workplace discrimination. They were stunned. Stunned that two conservative justices had sided with them. Stunned that this happened on top of a Republican appointee writing the marriage ruling, too.

In many ways, the decision is the strongest...