On Sunday night I interviewed Lyle Shelton, head of the Australian Christian Lobby, on Sky News about same-sex marriage.

Just two Christians – me and Lyle – having a chat about the role of religion in the public square, and about the nature of marriage, the teachings of Jesus and how we interpret the gospel. Also, we spent a bit of time disagreeing about whether “radical gender ideology” is the inevitable outcome of legalising same-sex marriage. Shelton says it is; I say it is not.

The ACL and others who oppose same-sex marriage have seized upon gender theory and programs like Safe Schools and conflated them with marriage equality. It is a gross misrepresentation to say that one is necessarily related to the other, or as Shelton claims, that legalising same-sex marriage may well mean that Safe Schools becomes compulsory in every school in Australia. It is a convenient straw man (person?) argument to introduce doubt and fear.

Frankly, if Malcolm Turnbull and George Brandis wanted to do one thing, just one thing, to help the marriage equality campaigners, it would be to strongly call this argument out for the sham that it is.

Shelton and I didn’t change each other’s mind on same-sex marriage. He still opposes it; I still support changing the Marriage Act to allow two people to marry, regardless of gender.

But it was an illuminating discussion nonetheless.

Firstly, it showed that the opposition some conservative Christian groups have against same-sex marriage is not just opposition to homosexual people accessing marriage, but rather presents one of the last remaining battlegrounds for them to stand against changes that are fundamentally altering the nature of heterosexual marriage.

For example, when Shelton argued that the state should not institute in public policy ways to create families where a child is “deprived” a mother and a father, I asked him about no-fault divorce.

“That’s a really good question,” he said, going on to explain that while he would never want people to be forced to remain in a marriage that was abusive, he would have preferred that the waiting period for a no-fault divorce had been set at two years so as to give people pause about the decision to end a marriage.

Shelton’s argument is that the shift from understanding marriage as a covenant to a contract has undermined the very idea of marriage as an enduring and stable basis for families in society.

There is an element of truth to that, if we are honest. But then again, it is folly to think that marriages were more idyllic and society was a better place back in the days when women and children had virtually no rights.

But why is denying same-sex people the right to marry going to undermine marriage as a “covenant”? If anything, granting same-sex couples the right to marry would strengthen marriage as an institution in society.

Andrew Sullivan, an American, Christian, homosexual and conservative writer, argues that extending marriage to same-sex couples strengthens the very notion of marriage as the basis for which families form. If these couples can’t access marriage, Sullivan argues, they will form families without marriage, demonstrating to heterosexual couples that cohabitating or civil unions are legitimate alternatives to marriage.

When it comes to whether homosexual couples choose to become parents, Shelton raises a range of concerns: that legalising homosexual marriage will lead next to a push to legalise commercial surrogacy, or spur the use of anonymous sperm or egg donors, and “deny” children the chance to have a mother and a father.

When I put to him that infertile heterosexual couples were already pushing for commercial surrogacy, or that we can and have created legal ways to ensure that children created through donor sperm or eggs have the right to know their biological parents, he acknowledged that these changes in how families are created – heterosexual or homosexual – have had massive changes to family relationships.

And that is what seems to really worry Shelton and those who share his views.

I suspect Shelton knows full well that changing the legal definition of marriage isn’t going to suddenly introduce these ideas and technologies. These “other ways” of creating children – surrogacy, IVF, donors – are already part of the fabric of heterosexual marriage. They are available to heterosexual couples (married of not) and homosexual couples (married or not).

Keeping the definition of marriage as between a man and a woman isn’t going to keep these technological changes at bay. They are already here. Same-sex marriage provides Shelton and others a chance to argue again against a contest they’ve already lost when it comes to heterosexual marriage.

We did talk about Jesus. It was quotes at 20 paces: me citing Jesus’ commandment to love one another, and to lay down one’s life for another, or Jesus’ description of his mother and brothers (his family) as not those who were biologically related but those who heard the call of God and responded in love.

For Shelton, it was Jesus saying that a man will leave his mother and a woman will leave her home and the two shall become one.

Jesus, I said, was a first century man in the Middle East. He had to reflect a certain culture and time. (I mean, if we lived exactly as Jesus did, I wouldn’t be able to enter the church when I was menstruating.) In short, as our understanding of humanity unfolds, so too does our understanding of God and how we are to live as God’s people.

For Shelton, Jesus’ rather specific words on marriage carry more weight. That is the nature of theology right there – attempting as humans to discern what is truthful and important in the gospels. Shelton and I aren’t the first theologians to disagree on how to interpret the gospels, and we won’t be the last.

But when I asked him if he accepted that there is a difference between a sacramental understanding of marriage and a legal definition, he demurred. There is, he says, but he believes they are so intrinsically related in our culture and human history that he doesn’t make such a distinction.

Lastly, we got on to an area of (sort of) agreement – religious freedom. Shelton is concerned that the plebiscite legislation does not spell out how religious freedom will be protected, and he wants to see florists and bakers able to refrain from providing services to homosexual weddings based on their religious convictions.

I don’t agree with Shelton that our laws should provide such exemptions, but I do agree that the Turnbull government must spell out what it proposes when it comes to religious freedom before asking Australians to take part in a plebiscite, or MPs to vote in parliament.

At Guardian Australia’s event Marriage Equality: Why Knot?, opposition leader Bill Shorten ruled out supporting the sorts of exemptions Shelton proposes. When I spoke to the communications minister, Mitch Fifield, last week on Sky News, he said he was certain the government would outline soon what it proposed in terms of religious freedom, and would do so before a vote on same-sex marriage. All of us, religious or not, should keep a close eye on them when they do.