On March 26th, the U.S. Supreme Court will hear arguments in Hollingsworth v. Perry, in which the respondents will broadly argue that state laws banning same-sex marriage (like California’s Prop 8 ) are unconstitutional under the Equal Protection Clause of the Fourteenth Amendment.

The Supreme Court will also hear a challenge to the Defense of Marriage Act (DOMA), meaning that two major Supreme Court cases argued this year will tackle the constitutionality of same-sex marriage bans.

And today, many prominent American businesses including Apple and Facebook have joined together to back same-sex marriage, publicly, in the form of amicus briefs filed on behalf of the cause.

In speaking against laws like Prop 8, companies including Apple, Nike, Facebook, Morgan Stanley, Intel, Xerox, AIG, Cisco Systems, Mesirow Financial, Oracle, Panasonic, Barnes & Noble, Office Depot, and Alaska Airlines argue that it’s not simply about fair treatment under the law, for constitutionality’s sake. They argue that there is a tangible, adverse effect on business that comes from these types of bans.

In addition to the compelling constitutional case, there is a very strong business case for recognizing the rights of same sex couples to marry. By singling out same-sex couples for unequal treatment, laws like Proposition 8 can impede business efforts to recruit, hire, and retain the best workers in an environment that enables them to perform at their best. Under Proposition 8, individuals in same-sex couples are denied the happiness and security that comes from marrying one’s loved one. This deprives those individuals from enjoying the many emotional, psychological, physical, and economic advantages that come from marriage—which may make content and satisfied individuals into happy and productive employees.

Proposition 8 also interposes an obstacle to recruiting and retaining the best and the brightest when those potential recruits or employees are members of a same-sex couple. Such individuals 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.

Of course, it’s not just about business.

“By enshrining in the law that gay men and lesbians are less worthy than heterosexual individuals—and that same-sex couples are inferior to opposite-sex couples—Proposition 8 announces that the law will tolerate discrimination against homosexual individuals. This message is absolutely contrary to amici’s belief in respect for, and fair treatment of, all people,” says the brief.

The amici curiae supporting the challenge of DOMA is signed by many more companies (over 200), and includes Apple, Facebook, Twitter, Zynga, Microsoft, Google, eBay, Intel, and Adobe.

In its summary, it also take the business angle:

Although marriages are celebrated and recognized under state law, DOMA, a federal law withholding marital benefits from some lawful marriages but not others, requires that employers treat one employee differently from another, when each is married, and each marriage is equally lawful. DOMA thus impairs employer/employee relations and other business interests. In this brief, amici show how the burden of DOMA’s dual regime is keenly felt by organizations that conduct operations or do business in jurisdictions that authorize or recognize marriage between two people of the same sex.

But there’s also the overarching argument that DOMA forces these companies to be compliant in discrimination that’s against their core principles.

“DOMA imposes on amici not simply the considerable burden of compliance and cost. DOMA conscripts amici to become the face of its mandate that two separate castes of married persons be identified and separately treated,” says the brief.