Maddie* had been married just over a year when she met the woman she would eventually cheat on her husband with.

Maddie was 19 and raising her first child. Her marriage, she says, was "difficult and complex".

"I was an isolated young mother without an enormous amount of support," she says.

"I met a woman in mothers' group who I became very good friends with, and over time, I fell in love with her."

One night, three years after they first met, their friendship tipped over into something physical. Not long after, Maddie told her husband about the affair.

"We were lying in bed and I buried my face in the pillow and kind of just, in a very small voice, told him," she says.

"The intensity with which I loved this woman and needed her was so much bigger than if I had all that support around me.

"We were so reliant on each other, and so interdependent in our being young mothers."

The affair, for Maddie, signalled the end of her belief in monogamy.

"I had slipped into this idea of the shape that I thought a relationship should be, the shape that a family should be, and I tried it out at such a young age," she says.

"What was revealed to me was that [marriage] didn't suit me, it didn't support me, and it wasn't what I wanted."

Female infidelity on the rise

Infidelity has existed since marriage was invented, says US psychotherapist and author Esther Perel.

But there's long been a double standard between the sexes when it comes to expectations of remaining faithful, Perel says.

"For so long in history … women were imposed upon to remain faithful and monogamous. Men have often practically had the license to cheat," she told ABC RN.

Today, women's increased economic independence, the possibility of leaving home, and changing cultural attitudes mean the infidelity gender gap is starting to close.

"Women have begun to express their desires in ways that are more similar to men," Perel says.

While the prevalence of infidelity is difficult to measure, US research suggests the number of women who cheat on their partners has risen by 40 per cent in the past three decades.

The rate of infidelity among men, however, has remained largely the same.

"Traditionally, we have loved to think that men cheat because they need novelty, they're bored and they like variety. And women only cheat because they're lonely and hungry for intimacy," Perel says.

"I think we will begin to see very different kinds of motives for why people stray once the consequences are equal for both genders."

Women held to a 'different standard'

Maddie's affair and the subsequent breakdown of her marriage remained mostly private.

But she says women, especially mothers, who commit acts of infidelity are held to a "completely different standard" — and therefore "vilified in a different way".

"I think more is expected of women in relationships, especially when they're mothers," she says.

"We don't give a lot of space for mothers to have a sensual, erotic life, so when women cheat, it's not only transgressing that partnership, it's transgressing what society expects women to be."

Speaking to ABC podcast Ladies, We Need To Talk, Brisbane psychologist Rebekka Sommer says the idea of women cheating is still seen as "a lot more confronting".

"Girls are raised to be good … a girl is taught that her value is in her being good, looking after a man, doing the right thing — almost as though she's somehow more immune to temptation than men," she says.

But Ms Sommer says infidelity is less about someone turning away from their partner and more about them turning away from themselves, or who they have become.

"There's often a sense of disconnectedness from self, and there's something about an affair that can almost feel like a bridge back to yourself," she says.

It's an idea Perel says is often highlighted among women who cheat — a sense that they have "lost themselves".

"They became full-time caregivers, mothers, wives … you name it. And they just didn't know how to stay connected their own erotic selves," she says.

"Why is it that when we become wife, mother and caregiver, we suddenly feel like we haven't been in the company of other parts of us for decades?"

A catalyst for change

Being unfaithful is usually not something women look for or expect, Ms Sommer says.

"Often something pops up — a co-worker or someone she meets — and she just feels really seen … in a way that she's not used to being seen," she says.

"That undivided attention is what I think often is the big appeal."

For Maddie, being seen and supported by her best friend — and going on to develop a relationship — was a key turning point to knowing she "needed something much bigger" than a nuclear family.

"Ten years on, I have built a community around me … I live with two other adults, and an extra child, and I have a community and support and I'm really well resourced," she says.

"Now my romantic relationships are a choice, for pleasure and for connection and for fun ... It's not that I need everything from somebody, it's more that I choose — if I feel like it — to connect with someone in that way."

Maddie says cheating, while creating positive change in her life, can be "extraordinarily hurtful".

"It does really blow up families," she says.

It is often, however, an indication that something needs to change, she says.

"I don't know if that means there needs to be a new relationship … I think it can be addressed with better communication, and a courageous revealing of needs. And that's a hard thing to do."

*Name has been changed for privacy reasons.