Australian prime minister Julia Gillard, who is personally opposed to marriage rights for gay couples, has told the father of a gay man that not all loving relationships need to be recognised as marriages.

Appearing on ABC’s Q&A programme, Ms Gillard told a citizen it was possible to have a “relationship of love, commitment and trust and understanding” without a marriage certificate.

Ms Gillard lives with her partner, Tim Mathieson, but the pair are not married. She has said she will allow her Labor party MPs a free vote on marriage equality legislation

Geoff Thomas had said to the prime minister: “I am a Vietnam veteran, and a plumbing contractor and a proud father of a gay son. Almost two years ago I asked Tony Abbott on Q&A when he would start treating gay and lesbian Australians with dignity and respect. He hasn’t changed his view, but you have.

“The country now knows that you are as one with Tony Abbott and the extreme Australian Christian lobby on this issue. Could you please explain to me how in a country that prides itself on freedom, equality and tolerance how it is that denying same sex couples the right to marry is in any way in any sense fair, just or consistent with the way Australians think today?”

Ms Gillard said it was possible to have a “relationship of love, commitment and trust and understanding that doesn’t need a marriage certificate associated with it”.

She said: “That’s my life experience and so I’m speaking from that life experience.

“If you believe as I do that people can have deep and committed relationships without a marriage certificate, it becomes an issue about how are we going to deal with this cultural institution of long standing in Australian society? Are we going to try and change it to fit circumstances where people are in love and deeply committed but don’t fit the current Marriage Act, or are we going to grow up new traditions and norms that embrace that?”

Ms Gillard said she held the view “very deeply” that marriage should not be opened up to gay couples, but said it was “not for her” to tell Labor party members how to vote on the issue in Parliament.

Mr Thomas told Ms Gillard had “abandoned the basic principle of treating every Australian as equal under the law. My son deserves to be treated equally under the law.”

The format of the programme meant Ms Gillard was not able to respond to his final comment.