Faith leaders who insist same-sex couples should not be able to marry — even those who also promote love and support for LGB people — may be causing serious harm to the mental health of LGB individuals, the author of a new study on the impacts of religious anti-gay prejudice has said.

LGBT people have frequently been shown in previous studies to have poorer mental health outcomes than their heterosexual counterparts, resulting in part from the stress that prejudice and anti-gay discrimination can cause.

But in the new study, published this week in the American Journal of Orthopsychiatry, researchers from Macquarie University found both LGB and heterosexual people who were exposed to even subtle religious anti-gay prejudice, such as disapproval of same-sexuality among religious groups, displayed higher levels of stress, shame, depression and anxiety.

LGB Christian participants exposed to religious anti-gay prejudice — for example, a Church stating its opposition to same-sex marriage — also reported experiencing greater conflict between their same-sex attraction and their religious beliefs.

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Many churches declined to respond directly to the study's findings but some told ABC News LGBT people must be accepted, and treated with love and respect.

But one of study's authors, Babucarr Sowe, said the findings should be a "wake-up call" for clergy and religious leaders, particularly given the current national discussion around same-sex marriage in Australia.

"When you have a prolonged national debate, injected with the staunchly anti-gay religious rhetoric that we're seeing now, the potential for harm becomes enormous," Mr Sowe told ABC News.

"When we're talking about a range of potential mental health, abuse, and substance use ramifications, the solution isn't ... simply "growing a spine'," Mr Sowe said, referring to comments by National Party Senator Matt Canavan, who last month said people who were offended by the same-sex marriage debate needed to "grow a spine" and "stop being delicate flowers".

Which churches oppose same-sex marriage?

The findings come as debate intensifies around Australia's same-sex marriage postal survey, the results of which will be announced on November 15.

The majority of Christian churches in Australia hold the position that marriage is between a man and woman.

The Anglican Diocese of Sydney last week announced it had donated $1 million to the No campaign against same-sex marriage, with Archbishop Glenn Davies saying the traditional understanding of marriage — "as a union of one man and one woman" — was a "positive good" for society.

(The move was criticised by senior Anglicans who questioned whether the money could be better spent on other social issues.)

However, the Anglican Diocese of Perth last week offered a "heartfelt apology" to LBGT people, saying the church was "deeply sorry for any harm we have done".

Some dissenting ministers from various denominations have also spoken out about the need to provide affirming spaces for those struggling to reconcile faith and sexuality.

Melbourne Baptist pastor, Simon Carey Holt, said he had surprised his church by indicating he would be voting 'yes' for same-sex marriage.

Baptists hold strongly to literal readings of the Bible, he said, and dissenting views are, in his experience, treated with anger and disappointment by mainstream congregants.

Pastor Holt said decades of ministering to same sex-attracted congregants had shown him the damaging impacts institutional disapproval can have on those struggling to reconcile faith and sexuality.

"We're dealing with people who are pretty battered and vulnerable around the intensity of the debate that is going on," Pastor Holt said.

ReachOut, a mental health provider for young LGBT Australians and their parents, said demand for its support services had spiked 20 per cent during the postal survey period.

"We're certainly hearing things around young people saying 'am I a freak?', 'will I be accepted?'" ReachOut CEO Jono Nicholas told ABC News last month.

'Hating the sin' of homosexuality may undermine pastors' mental health obligations

The four-year study surveyed 1,600 individuals in the United States across a range of metrics including mental health, the level of perceived conflict between religion and sexuality, social support, experiences of abuse as well as drug and alcohol use.

(Researchers used an American sample in order to access a broader demographic and limit selection bias.)

Respondents were also invited to rate perceived or encountered disapproval of same-sex sexuality from family, friends, their school and workplace, as well as faith communities.

It found that religious anti-gay prejudice also negatively impacted heterosexual individuals (many of whom have gay friends and family).

ReachOut said demand for its LGBT mental health support services had spiked 20 per cent during the postal survey period. ( ABC News: Margaret Burin )

"Homonegative religious exposure may be of greater health and mental health concern than is conventionally recognised," the authors wrote, "potentially undermining the wellbeing of both religious and non-religious LGB persons as well as their heterosexual counterparts."

Mr Sowe said the study also broadened the scope of what constituted damaging prejudice.

It was not only abuse and hostile forms of anti-gay aggression that could be problematic, he said, but subtle forms of prejudice, too, including anxiety experienced by the expectation of rejection, and environments in which same-sexuality is considered wrong (for example, in many churches).

Importantly, he said, attempts by clergy and faith leaders to demonstrate love and tolerance toward homosexuals while continuing to "hate the sin" of homosexuality "may undermine the objectives and mental health obligations" of religious pastoral care.

"Hopefully these findings encourage supportive religious people to continue speaking up [against anti-gay prejudice] because their advocacy in countering everyday anti-gay religious messages may have broad and significant benefits."

Church has an 'opportunity to embrace' LGBTI people, pastor says

The head of the Anglican Church in Australia, Archbishop Philip Freier, said that while the Church is welcoming of LGBTI members, its official position is that marriage is between a man and a woman.

"We recognise that individual Anglicans vary widely on their vote and their reasons, and we encourage them to exercise their own conscience," Archbishop Freier said in a statement to ABC News.

"We also recognise that the Anglican Church has not always dealt with LGBTI people with the love with which we are called to treat all human beings, and this is a matter of shame and regret."

Sorry, this video has expired Catholic Archbishop of Sydney delivers homily on same-sex marriage

A spokesperson for the Catholic Archdiocese of Sydney declined to comment specifically on the study's findings but said:

"The Catechism of the Catholic Church calls for understanding for those with deep seated homosexual tendencies for whom this may well be a major trial. They must be accepted with respect, compassion and sensitivity.

"The Catholic Church opposes all forms of unjust discrimination. We deplore injustice perpetrated by anyone because of religion, race, sex or age."

Melbourne Uniting Church Minister Dr Avril Hannah-Jones, who is openly bisexual, said the study's findings reaffirmed the need for religious bodies to rethink their attitude towards LGB congregants.

"The single most important thing that religious bodies can do is stop saying that being LGBTIQ is somehow a mistake in God's creation," Dr Hannah-Jones said.

"Until churches can say that LGBTIQ people are part of the rich diversity of God's good creation and that God created us to be who we are and loves us just as we are, then those of us who are religious will continue to suffer psychologically."

Pastor Holt, who leads the Collins Street Baptist Church in central Melbourne, said conducting funerals of young LGBT people who "felt shame and guilt in their identity" and had committed suicide had been "confronting" and had inspired him to review his readings of scripture.

"I have come to a place that these issues of sexual identity run very, very deep. This [their sexuality] is not choice or lifestyle option, this is who they are," Pastor Holt said.

Many of his congregants were "profoundly shaped by religious tradition", he added, and as a result struggled with issues that challenged their interpretation of faith, such as the current debate around same-sex marriage.

But, he said: "It's possible to see sexuality in a broader and welcoming way".

"The Church has an opportunity to embrace and support and walk alongside these people ... in the way that leads them into the fulfilment of life rather rather than despair and guilt and shame."

The Catholic Archdiocese of Brisbane declined to comment for this story.