(Read More: Corporate Call for Change in Gay Marriage Case)
The Human Rights Campaign has found far greater commitment among corporate leaders focused on their bottom lines than among politicians seeking votes.
In 29 states it remains legal to fire or not hire someone on the basis of sexual orientation.
But among the 688 major employers rated in its most recent survey, HRC found that 99 percent prohibit discrimination based on sexual orientation and 89 percent offer health benefits to domestic partners.
In addition, 65 percent have parity in "soft" benefits for spousal and domestic partners such as bereavement leave, employee assistance programs, employee discounts, and relocation assistance.
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Among the 20 largest publicly-traded firms on the Fortune 500 list, 13 received a perfect 100 percent rating across a range of practices relating to gay rights. Those include: Chevron, General Motors, Bank of America, Ford, Hewlett-Packard, AT&T, JPMorgan Chase, Verizon, AIG, and IBM.
"The private sector was always ahead of the politicians," said Hilary Rosen, a Washington public relations consultant active in gay-rights causes.
Now the public opinion has shifted markedly—driven in large part by younger voters who don't even regard gay marriage as controversial—politicians are catching up.
(Read More: Gay Rights: Does Taking a Stand Affect Business?)
President Barack Obama embraced gay marriage in his 2012 re-election campaign. Sen. Rob Portman, R-Ohio, recently announced his support for gay marriage after learning his adult son is gay.
The dramatic increase in the number of gays and lesbians open about their sexuality has been a key element in the shifting dynamics of the issue. Whatever the Supreme Court rules, all signs suggest that both the public and American business will increasingly regard marriage rights as a settled issue.
—By CNBC's John Harwood; Follow him on Twitter: @JohnJHarwood