Social media giant Facebook has found itself in hot water after it endorsed same-sex marriage in Australia, which is due to vote on the controversial issue.

Starting this March, all 14 million Facebook users in Australia have been given the option of adding a pro same-sex marriage rainbow slogan at the bottom of their profile pictures. They can add a "frame feature" reading "I [heart] Marriage Equality." Facebook created this artwork in partnership with the group Australian Marriage Equality (AME).

While same-sex marriage supporters appreciate this feature, a lot of Christians are not pleased at all because Facebook did not provide a slogan for those who oppose same-sex marriage.

Lyle Shelton, managing director of the Australian Christian Lobby, said Facebook is entitled to support whatever cause it wants because it is a shareholder owned company. However, she said Facebook should also consider that many of its users "will always believe marriage is between one man and one woman."

"Many people will feel pressured not to express their views in support of marriage because of Facebook's enabling of just one side of the debate," she told the Sydney Morning Herald. "The beauty of Facebook is that it has been a forum for free discussion. This move will make many people feel less free to engage in an important public debate."

Meanwhile, Facebook director of policy for Australia, Mia Garlick, said marriage equality is "one of the most talked-about topics on Facebook." The frame feature, she said, was just "a way of us amplifying the conversations that we already see happening on the platform."

AME campaign director Erin McCallum also lauded Facebook's support, saying the new feature is a "simple, yet powerful way for individuals and businesses to show what we already know."