EXCLUSIVE: Christine Forster writes “I will never agree with Tony”.

Christine Forster met her partner Virginia Edwards 13 years ago when they had sons at the same school. Coming out and becoming a couple was a huge decision for both women, who were both married when they met.

Forster is former PM Tony Abbott’s sister and Liberal Party Councillor in the City of Sydney. She disagrees completely with her brother that gay marriage will “erode” conventional marriage and strongly believes everyone has the right to create their own family – whatever that family may look like.

Christine writes exclusively for Mamamia on the issue that has divided them…

Last week my older brother, former Australian Prime Minister Tony Abbott, gave a speech in the US which he titled the “defence of families.”

He and I have a well-documented difference of opinion on whether or not Australian legislation should be changed to enable two men or two women to marry.

Tony has again clearly put his view that the “essence of marriage” is “one man and one woman open to children.” In contrast, it’s my firmly held belief that the legal right to marry one other person, as it is defined by government legislation, should be extended to all men and women, irrespective of the gender of their chosen partner.

Like Tony, I believe that marriage is a special relationship. That’s why it forms the bedrock of our society and why governments have enshrined rules around marriage into legislation.

I also wholeheartedly agree with my brother when he says that marriage is “the heart of family.” Where we differ fundamentally is that I regard marriage as being first and foremost about two people becoming family to each other.

It’s my view that people, both heterosexual and homosexual, primarily want to be married because they share a bond that is “family” by definition.

Listen to Christine tell the Just Between Us podcast about coming out later in life. Contrary to popular perceptions, her brother Tony Abbott was the most supportive of all: (post continues after audio)

As Tony described it in the US last week: “With family, we don’t just return phone calls, try to remember birthdays and arrange to meet when we haven’t caught up for a while. Family are the people whose lawn we will mow, or whose dishes we will wash, or whose debts we will forgive, or whose death bed we will attend – not because we necessarily feel like it or because it might do us good – but because they have a claim on us: of blood or of deep, long-term connectedness.”

People want to get married when they find that special person, who is not a blood relative, but whom inspires that depth of connectedness. When two people decide to marry, the very act of their wedding says unequivocally to their families, friends and communities that “we two regard each other as family, to the exclusion of all others.”

And in my experience, the desire for and the occurrence of those special relationships is just as common for gay people as it is for straight people. We are all human, after all.

But it’s my fierce belief in the unique nature of marriage and how it binds us as people and societies that compels me to challenge Tony’s suggestions that changing the Marriage Act in Australia would lead to the “erosion” of the family.

“We can’t shirk our responsibilities to the future; but let’s also respect and appreciate values and institutions that have stood the test of time and pass them on, undamaged, when that’s best,” my brother said.

Exactly how does it “erode” or “damage” the marriages and/or families of heterosexual Australians if their gay neighbours are able to exercise the right to marry before the law?

In fact, we’ve already tested his premise, both here and overseas, and proved it to be unfounded. There is no evidence that I’ve heard of that there has been any impact on marriages between men and women in Canada, the UK, New Zealand or the US as a result of those jurisdictions legalising same-sex marriage. Nor did anything change for anyone else when 31 Australian couples wed under the short-lived ACT law allowing same-sex marriages in December 2013.

The reality is that people want to marry because they value the institution, not because they want to diminish it. I will never agree with Tony that my getting married to my partner Virginia would or could have any impact on anyone else’s family or relationships. Our marriage, therefore, would have none of the “consequences for the wider community” that concern him.

Where I can wholeheartedly concur with my brother, though, is on how the debate on this issue should be conducted: with common sense, patience, respect and engagement.

This will be particularly important if the outcome is ultimately to be decided by a plebiscite, which will inevitably involve heavy campaigning for both the yes and the no case. Those campaigns and the discourse that will surround them will unfortunately come with the potential to cause deep divisions among us.

As Tony said: “My plea, to everyone who wants a better world … is to place ourselves in others’ shoes and to keep conversation civil so that we can better understand even if we can’t quite agree.”

That will be crucial as Australia moves, I hope, toward my vision of a “better world.”