"It is important for people to ensure their voices are heard on this vital issue," the ACL wrote in a statement on Tuesday.
"We believe [traditional marriage] is good for society as it is the core institution of the family and justice for children."
If passed, the proposal by the Greens party would mean that same-sex couples married overseas would have their marriages recognised in Australia.
This is not the first time a proposal recognising such unions has been put forward. Last year an almost identical bill was voted down 44-28.
At the time Liberal Senator Sue Boyce, who crossed the floor to vote for the bill, labelled it "a backdoor way to try to increase support for same-sex marriage in Australia."
"This bill is a sneaky, backdoor attempt to try and import same-sex marriage to Australia and is intended to put increasing pressure on Australian politicians to redefine marriage," the ACL said.
"Some overseas jurisdictions recognise polygamous and even child marriages. Should Australia provide 'marriage equality' for these too?" the group asked.
Meanwhile, opposition leader Bill Shorten, who is set to address the ACL's national conference later this year, has been urged to explain why he supports same-sex marriage.
"Bill Shorten should remind the ACL conference that, like the majority of Australians, including Australian Christians, he supports marriage equality," Australian Marriage Equality national director Rodney Croome told Gay News Network.
"As someone who has voted to give equal recognition and protection to same-sex parents and their children, I'd also like to see him explain that love and not gender makes a family," Croome said.
The submission to preserve traditional marriage in Australia closes on July 31, and can be found at preservingmarriage.acl.org.au.
