When I was a teenager I used to declare that “marriage is an empty constitution”. I had misheard someone say “institution” once and, thinking it sounded grand and clever, I repeated the phrase ad nauseam for years.

Then I hit my mid-20s and the wedding invites started flooding in, then as if on cue I sent out my own.

In Australia, the average age for first-time marriage has risen to 29.9 for women and 31.9 for men. But still, millennials are getting married.

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We have watched our parents’ marriages disintegrate (well, half the time), been raised as feminists aware of the oppressive history of the wife, raged against the exclusion of our queer brothers and sisters, and become largely agnostic – but still, millennials are getting married.

According to the last census, the marriage rate is actually on the rise – but in 2018, what possible reason is there for that? It’s a question that was rattling around my head for so long I ended up writing a novel about it; a book is a great way to ask people why they got married without sounding terribly accusing.

My friend Caro married her longtime boyfriend before marriage equality was legalised and not only regrets the decision, but struggles to articulate why she did it.

“I was almost a bit embarrassed we were getting married considering how strongly I identify with my progressive beliefs,” she says. For her, it was a matter of ticking boxes.

“I didn’t want to have kids, I didn’t want to own a home, and I felt I needed to tick one of those boxes before I was 30. He proposed just before my 30th birthday.

“The way society is structured, and the stories told from when we were little – if we don’t tick one of those boxes we feel like less of an adult.”

It’s a theory the Australian writer Briohny Doyle explores in her book Adult Fantasy. “The sustained perception of marriage as a transition from the juvenile to the adult was brought home in a gross and practical way for me after I got my sparkler,” Doyle writes of her engagement ring. “When wedding plans faded, the ring became a prop. I wore it to job interviews, other people’s weddings, when speaking to the neighbours about our barking dog or loud music. It was a little bit of shiny proof I was a legit adult, a person of serious intents.”

Millennials like me are still being written about as wide-eyed youths, but many are in their mid-30s now. We are thinking about how we are getting old and what ageing is going to look like for us. Most of us spent our 20s working for free or in the insecure gig economy, or getting a good job and then being stuck in the same pay bracket for years. In a property market on steroids, that leaves us with not many ways to feel secure. If you’re in a loving relationship, it’s the last raft you can cling to as you start staring down middle age. If you get married you can tell yourself at least someone loves you who is not your mum. So perhaps that’s part of it.

But then someone can love you and be committed to you for life without signing a legal document. Gay couples in Australia have been forced to conduct their relationships that way until recently. But marriage – that official paperwork – still holds an allure.

When talking to my friends about why they decided to get married in general, it seemed a lot more simple for the men. Caro says her partner pushed for the marriage to be a legal one. For reasons neither of them could really explain, it made him feel more secure if the marriage felt more “real”.

Another friend held a commitment ceremony with his boyfriend before marriage equality was legalised; after that, they made it legal.

“I wanted James and I to stand in front of the people we love and formalise our commitment to each other. I, maybe naively, feel like [getting married] makes you work harder to get through the tough times and is a constant reminder of how much you love or loved each other,” he says. “I think it was also, for me particularly, a desire to just be like everyone else; to feel included in something I see as the ultimate expression of love. It’s corny but I love love, and getting married is the pinnacle of that.”

Another friend, Lucy, married her girlfriend in a big wedding that was not legally binding; she found herself single just as marriage equality was passed in Australia. She was young when she married – 25 – and looking back said her main reason was to legitimise her relationship in the eyes of others.

“If I were straight I would have been way less inclined to do it – there is the conservative nature of marriage as an institution – but it seemed less expected for me to do it, so I wanted to do it,” she says.

“Honestly it was about the romance, the gesture. I don’t just mean ‘love’ romance; I mean it being a big statement, a big movement in your life, a big change – which it ultimately wasn’t. I didn’t want to tell people I was married because I felt a little ashamed it wasn’t ‘real’, but also I felt like a pioneer. I romanticised it in that way, having made this statement.

“I also felt like I was representing all gays ... I needed to prove we can do it better than straight people.”

There are plenty of legal advantages to getting married. Although people argue that de facto couples have the same rights in Australia, it’s actually quite difficult to prove de facto status – and in extreme circumstances, say the hospitalisation of your partner, it’s much easier to walk in and say “that’s my wife”. But most millennials aren’t making the decision to marry because they are clear-eyed about the legal protections; for most, the reasons are far more complicated.

If we are being brutally realistic, many marriages are probably not going to last. But the Great Barrier Reef is not going to last either, and we don’t know how much of the world is going to last if Trump and North Korea keep playing chicken. Perhaps getting married is an act of supreme optimism in what is the most anxious age to date; something to try to give solid footing to a generation who have been mainlining the internet and its accompanying anxieties for their entire adult lives.

• Bridie Jabour’s The Way Things Should Be is out 1 May through Bonnier Publishing