Momentum is building behind the push to legalise same sex marriage with Prime Minister Tony Abbott being labelled Australia's "biggest roadblock" to marriage equality.

Senator Penny Wong also slammed some of the comments around the issue, labelling them "illogical and outright offensive".

Asked whether she was stifling debate on same sex marriage, Labor Senator Wong referenced two Liberal senators strongly opposed to the idea.

"I didn't notice Eric Abetz being stifled in any way. I never noticed Cory Bernardi being stifled in any way," she told reporters on Saturday.

"The sort of language and propositions from those who oppose marriage equality flick between illogical and outright offensive."

Government frontbencher Senator Abetz on Friday defended penning a sternly-worded rebuke to Hobart City Council for expressing support for same sex marriage.

In the letter, dated June 17, the Tasmanian senator said the council's resolution "undermines the important social institution of marriage" and had nothing to do with the priorities of ratepayers.

Senator Abetz earlier suggested frontbenchers who supported change should resign, and that allowing same sex marriage would open a "Pandora's box" and potentially lead to polyamory.

Senator Wong said she wished the standard of the debate would be a little better.

Earlier Labor MP Joel Fitzgibbon said he'd changed his mind on same sex marriage after voting against it three years ago and believed the reforms would pass parliament if Tony Abbott allowed a free vote.

"He's using his power over the party room to represent the case of a shrinking minority over the case of a growing majority in the people's house," Mr Fitzgibbon told Sky News.

The National Rugby League has also thrown its support behind the push for same-sex marriage.

Chief operating officer Suzanne Young said inclusiveness was a core value of the NRL.

Liberal MP Warren Entsch is proposing to take a same sex marriage private member's bill to parliament in August, co-sponsored by Labor MP Terri Butler.