FORTUNE — On Thursday, dozens of American corporations, including Apple, Alcoa, Facebook, eBay, Intel, and Morgan Stanley will submit an amicus brief in the landmark Hollingsworth v. Perry case broadly arguing to the U.S. Supreme Court that laws banning same-sex marriages, like California’s ballot initiative Proposition 8, are unconstitutional under the Due Process and Equal Protection Clauses.
According to a draft copy obtained by Fortune, the companies argue that such laws “send an unmistakeable signal that same-sex couples are in some way inferior to opposite-sex couples, a proposition that is anathema to amici’s commitment to equality and fair treatment to all.”
At least 60 companies had committed to signing the brief as of Tuesday evening, according to Joshua Rosenkranz, who is counsel of record on the brief and head of the Supreme Court and appellate litigation practice at Orrick, Herrington & Sutcliffe. That number is expected to rise by Thursday, however, according to Rosenkranz. Others who have already committed to sign include AIG, Becton Dickinson, Cisco, Cummins, Kimpton, Levi Strauss, McGraw Hill, NCR, Nike, Office Depot, Oracle, Panasonic, Qualcomm, and Xerox. (Update: Verizon and Cablevision have now joined.)
Though the brief adopts by reference all of the arguments of the main brief challenging the constitutionality of Prop 8 — filed last week by Theodore Olson of Gibson Dunn & Crutcher and David Boies of Boies, Schiller & Flexner — it also argues that “recognizing the rights of same-sex couples to marry is more than a constitutional issue. It is a business imperative.”
Fleshing out that contention, the companies argue: “By singling out a group for less favorable treatment, Proposition 8 impedes businesses from achieving the market’s ideal of efficient operations — particularly in recruiting, hiring, and retaining talented people who are in the best position to operate at their highest capacity. Amici are competing domestically and internationally with companies inside and outside the United States in places where all couples, regardless of whether they are of the same sex, are afforded equal access to marriage.”
Specifically, it suggests that when “potential recruits or employees are members of a same-sex couple,” they “may forgo the opportunity to work in California, and prefer other states (like Iowa, New York, and Massachusetts) or other nations (like Spain, Sweden, Denmark, the Netherlands, Portugal, or Belgium) where they can be married and obtain equal treatment and respect under the law.”
Laws like Prop 8 “leave companies in the untenable position of being compelled implicitly to endorse the second-class status to which their gay and lesbian employees, clients, customers, and business associates are relegated,” the brief argues. “Until the law no longer relegates same-sex couples to second-class status as inferior “domestic partnerships,” our adherence to the law compels us to abide by a distinction that stigmatizes and dehumanizes gay men and lesbians.”
In an apparent effort to avoid giving offense to customers, vendors, and employees who may favor prohibitions against gay marriage, the brief states in a footnote that the signatory companies “do not mean to suggest” that such laws are the “product of ill will” on anyone’s part.
Finally, leaving no stone unturned, the draft brief argues that “increased wedding celebrations can mean additional revenue for many businesses — such as businesses involved directly in wedding celebrations, businesses that produce goods often given as gifts to newlyweds, and businesses that benefit from increased tourism from guests who travel to the wedding.”