The Law Council says exempting civil celebrants from being forced to conduct gay marriages discriminates against same-sex, intersex and transgendered couples.
The Turnbull government is being urged to drop a proposal that would allow civil celebrants and religious service providers to refuse to take part in same-sex marriages.
Peak bodies for Australia's legal profession and civil celebrants told a Senate inquiry on Monday there was no need to include the contentious exemption in draft laws legalising same-sex marriage.
Both, however, support keeping existing protections in the Marriage Act that make it clear ministers of religions aren't obliged to solemnise any marriage.
The Law Council argued the exemptions discriminated against same-sex, intersex and transgendered couples "without any proper basis".
President Fiona McLeod SC said civil celebrants perform secular - not religious ceremonies - and don't merit the same protection of religious freedom.
The Coalition of Celebrant Associations, which represents almost all of the country's 8000-plus marriage celebrants, agrees.
Vice-chair Liz Pforr said celebrants already can - and do - refuse marrying couples for various reasons, sometimes by simply saying they're not available.
"Our objection is that it is just same-sex couples that are being targeted with this bill," she told a hearing of the inquiry in Melbourne.
If exemptions were to be included, they should only apply to existing celebrants and those who are made exempt should be signposted, she said.
A recent survey by the group found 80 per cent of celebrants would be happy to marry same-sex couples, just over 10 per cent would discretely refuse while three per cent would resign if forced by law to accept it.
Margaret Mayman, an openly gay minister at Sydney's Pitt Street Uniting Church, said there was no reason to enshrine into law reasons to refuse a marriage.
She has refused to marry a couple where she knew there was ongoing physical and emotional abuse under existing religious freedoms.
"We do not support extending exemptions beyond religious officiants," Reverend Mayman said.
A group of conservative lawyers wants even broader exemptions.
The Wilberforce Foundation believes everyone should be allowed to refuse to provide goods and services to a same-sex couple based on their conscience and not just religious beliefs, just like ministers of religion and celebrants under the draft laws.
The inquiry, which is also holding public hearings in Sydney and Canberra this week, comes despite the Senate killing off the Turnbull government's proposed plebiscite on the issue.
It's not dealing with the plebiscite but rather the bill which would be introduced to parliament following voter approval.
The committee won't be re-examining the debate for and against same-sex marriage.