Sydney - Australia's prime minister on Friday defended a church's right not to marry some couples following reports a woman's wedding was cancelled after she posted support for same-sex marriage on Facebook ahead of a national vote.A voluntary postal ballot involving up to 15 million Australians on whether gay marriage should be legalised is underway, with the results due in mid-November.The survey is non-binding but Prime Minister Malcolm Turnbull has vowed to hold a vote in parliament if the majority of Australians choose "yes".The poll has attracted heated debate from the "yes" and "no" campaigns, with some prominent clergy speaking out against such unions, warning that legalising them could infringe on religious freedom.Fairfax Media reported on Friday that one couple were due to be married in the rural town of Ballarat in Victoria, but after the bride-to-be posted support for changing marriage laws on Facebook she was told the church minister would no longer officiate."You must surely appreciate that your commitment to same-sex marriage opposes the teaching of Christ Jesus and the scriptural position practised by the Presbyterian Church of Australia and by me," the minister wrote in a letter to the bride provided to Fairfax."This conflict of views has practical consequences in relation to your upcoming wedding. By continuing to officiate it would appear... that I support your views on same-sex marriage or that I am uncaring about this matter."The church, Ebenezer St John's, had no immediate comment on Friday, but Turnbull defended the minister's decision, saying "churches are free to marry whoever they like".