Controversial amendments to the same-sex marriage legislation are poised for defeat in Parliament this week, with key crossbenchers and Liberal MPs refusing to back them.

As debate began in the House of Representatives on Monday, supporters of the Dean Smith bill believed they had enough votes, 74, to form a blocking majority against any changes.

Prime Minister Malcolm Turnbull said gay people were better off living together than alone with their cats. Alex Ellinghausen

Conservative government MPs Michael Sukkar and Andrew Hastie presented nine tranches of amendments to colleagues late on Monday, including wide-ranging exemptions for civil celebrants, small businesses and organisations such as religious charities.

In a briefing, they told fellow Coalition MPs that anti-discrimination laws were "significantly unbalanced" toward protecting "same-sex oriented Australians", and offered "no protection at all" to those who support the traditional definition of marriage.

Former prime minister Tony Abbott said he would not oppose the same-sex marriage bill. Dominic Lorrimer

In addition, Mr Sukkar and Mr Hastie want the Marriage Act to "separately recognise both man-woman marriage and two-person marriage" – a definitional quirk which critics parodied as "separate but equal".

The proposed amendments, seen by Fairfax Media, largely regurgitate those that were emphatically rejected by the Senate last week.

Labor, along with Liberal MP Warren Entsch and crossbenchers Adam Bandt, Rebekha Sharkie and Andrew Wilkie, have 73 of the 74 votes necessary to block amendments in the lower house.

Several other Liberal MPs who spoke to Fairfax Media on Monday indicated they would also vote down the proposed amendments, though they would not say so publicly before reading them.

Liberal MP Tim Wilson is embraced by colleague Trent Zimmerman after his speech. Alex Ellinghausen

Key crossbencher Cathy McGowan, who has been aggressively lobbied in her regional Victorian seat of Indi, told Fairfax Media she would study all proposals but "they would have to be good arguments" to win her support.

Financial Services Minister Kelly O'Dwyer has also declared she is unconvinced by the amendments proposed so far. If any amendments were successful, the bill would need to return to the Senate, compounding delays caused by a lengthy speaking list.

Liberal MP Trent Zimmerman looks at Warren Entsch's rainbow tie. Alex Ellinghausen

In his contribution, former prime minister Tony Abbott – an avid opponent of same-sex marriage – conceded the fight had been lost and said he would not oppose the bill. He said he looked forward to attending the wedding of his sister, City of Sydney councillor Christine Forster, to partner Virginia Edwards next year.

Prime Minister Malcolm Turnbull said there was "sincere, heartfelt anxiety" the bill would infringe upon religious freedom, and therefore he would back multiple changes to give "additional reassurance in respect of these fundamental rights".

In his speech, he declared his support for same-sex marriage was about "commitment" as well as equality, and asked: "If we believe two gay people are better off together than living alone, comforted only by their respective cats, then why should we deprive that relationship of equal recognition?"

Mr Turnbull also noted he was the only Australian prime minister to be "unequivocally and consistently in support of legalising same-sex marriage", and said it would be "forever to the credit of the Coalition that this momentous social change occurred with the overwhelming mandate of the Australian people" through the postal survey.

However, new evidence emerged on Monday of the negative experiences endured by members of the lesbian, gay, bisexual, transgender, intersex and queer community during the postal vote.

A survey of 9500 LGBTIQ people by the left-leaning Australia Institute and the National LGBTI Health Alliance identified higher levels of personal stress and trauma than are widely appreciated. There was also anecdotal evidence of increased verbal and sometimes physical assaults.

The survey showed more than 80 per cent of LGBTIQ people found the survey stressful, and as many as seven out of 10 people in the affected community avoided being with people in general as a way of coping.

Australia Institute deputy director Ebony Bennett said the postal survey result was "overwhelmingly positive politically" but the debate "has taken a real toll on the LGBTIQ community".

Also on Monday, the Bureau of Statistics revealed a 10 per cent rise in the number of same-sex couples in Australia since last year. There are now 57,900 such couples, including more than 7000 with young children.

The self-declared figure represents just 1 per cent of Australia's 5.7 million couples, but is likely to significantly under-count the actual number of same-sex attracted Australians.

- with Mark Kenny and Peter Martin