Corporate giants are coming out in support of same-sex marriage as part of a letter-writing campaign to increase pressure on Federal Parliament to pass the reform.

Organisations including Qantas, PricewaterhouseCoopers, SBS and Football Federation Australia have signed an open letter that argues same-sex marriage is not just "the only truly fair option" but "a sound economic option" - as it will keep workforces happy and therefore productive as well as competitive for talent worldwide.

FFA chief David Gallop says his sport embraces same-sex marriage. Credit:Getty Images

FFA chief executive David Gallop told Fairfax Media: "A sport like ours is uniquely placed to raise awareness about this kind of social issue. And we take seriously our support for diversity and inclusion."

Over the past decade, organisations such as Telstra, Optus, ANZ, Westpac, IBM and David Jones have indicated their support for same-sex marriage.