Much of Shelton's world view seems to be informed by his beliefs on sexuality. He believes that a person's sexuality is about the physical act of sex and that everyone has some level of choice over their sexual preference.

Couched in discussion of the rights of children to a mother and father is Shelton's unshakable belief that same-sex attracted people are broadly not suited to raising children and that much of this debate comes down to a lifestyle choice.

"There's no scientific evidence to prove that. I'm not saying that gay feelings are not very, very powerful. They obviously are. But I don't believe they're innate. And the fact that many people have periods of their life where they feel attracted one way sexually and then another way. And the fact that you have "LGBT...I..." the whole range [of letters in LGBTIQ], the whole gamut. I don't think that it's something that you could say is innate. And certainly there's been no scientific evidence to that effect," he says.

"We all have strong and powerful desires. All of us. I think if you accept that argument, [that sexuality is innate] then yes, the debate would essentially be over."

"But I don't think that's right at all. And the fact that it is so fluid for so many people, then that isn't the basis on which to make public policy which affects children," he says.

Shelton believes the battle for same-sex marriage is more about the LGBT community seeking mainstream legitimacy than entering into the institution of marriage.

"I think it's part of the whole gay political agenda... it's not necessarily about marriage, it's about acceptance, and I think there are other ways to achieve acceptance rather than to change public policy on marriage," he says.

"Our acceptance of anyone who is same-sex attracted is not based on their sexuality it's based on who you are as a person.

"If we've failed as a society", he says pensively, "and if Christians have failed and there has been a failure to accept who you are… I don't think any of us are defined by our sexuality. We're defined by who we are as human creatures. That's where I think both sides of this debate have been wrong in the past.

"I think that's where we've failed in the past, we haven't accepted each other on that basis and I think that's the starting point of our discussion and that's where public policy should flow from – our anthropology of who we are as creatures, not as we're defined by our sexuality."

Ultimately, Shelton concedes that the battle against same-sex marriage is one the ACL is likely to lose. It's more a question of when, not if. But he remains hopeful that Australia can have a deeper, more meaningful debate before that happens.

"I'm an optimist," he says. "Certainly the signs don't always look good. But I believe if the public have the opportunity to think deeply about this debate, I think the truth about the needs of the child, if that's raised in the consciousness of the public, I think we'll see that this is something we do need to protect as a cultural norm."

"I suppose you've got to believe. If you believe that what you're doing can make a difference for people, that gives you the motivation to keep going."

Shelton knows he is fighting the tide of popular opinion, but he also knows that in the end there are only a handful of people who will actually need to vote for reform.

"With legislators, they often have to do the hard thinking that the general public don't. You can even be asked the question by a pollster, but if you've got to sit down and write public policy and think about the consequences of changes for freedom of conscience and the rights of a child, all these things that come into play.

"I think those are the reasons we see parliamentarians resistant to this change. Because they know that there are consequences to this. And often the public are a bit behind that, and need to be brought on that journey."

As polling consistently shows that a majority of Australians now back marriage reform, Shelton's sphere of influence in this arena continues to shrink. Increasingly, he needs to talk not to mainstream Australia, but to the 226 members of the Australian parliament. These are the people who ultimately will decide whether or not gay and lesbian Australians are worthy of the institution of marriage.

And in the end, those members are the only people Lyle Shelton needs to convince.