After the 1989 blockbuster movie “When Harry Met Sally…” many were left questioning whether truly platonic relationships are possible. But friendships between men and women really do exist and, if anything, are becoming increasingly common.

Over the past two decades, the differences between male and female societal roles have narrowed. Women are spending time in the workplace beside men; men are more actively participating in child care, housework and parenting. These generational shifts have spawned cross-gender friendships that your grandma never dreamed about.

Yet there is still a paucity of research and no roadmap to guide us in handling these complicated relationships. That’s why we tend to resort to shorthand when explaining them. We may say having an opposite-sex friend is like “having a sister” or “having a brother.”

Shared values and expectations are essential to any friendship, but achieving this between platonic friends is especially tricky.

Kate’s experience highlights the potential pitfalls of failing to define a platonic friendship explicitly from the beginning — and, perhaps, redefining it periodically. (The names here are not real.)

The case of Kate

Kate met her best friend, Jake, through her husband, Marc. At first, the three friends went to the movies and had dinner together quite often. Then Jake met Allyson, who would later become his wife, and the threesome became a foursome.

Jake was outgoing and loved being with people. Allyson worked long hours as a nurse, so Jake fell into the habit of coming over often to hang out with his old buddies. “It began to seem like he was always with us,” Kate says. He was even there, clapping like an uncle, when her son, Ari, took his first steps.

If Marc was working, Jake would accompany Kate to the mall to pick out baby clothes or to shop for gifts. “Jake actually enjoyed shopping, and I often joked he was my favorite girlfriend,” Kate says. Marc trusted his best friend (and his wife) so there wasn’t a hint of jealousy.

“There was only one time when I felt really awkward with Jake,” Kate admits. The two couples were at a movie, and Jake asked his wife to switch seats so he could sit beside Kate. “I nearly died,” she says. “I was so embarrassed and stunned he would do something like that.” But that seemed like a one-time gaffe.

A few years into their friendship, Jake confided to Marc that his marriage was foundering. Kate and Marc encouraged Jake to see a marriage counselor and tried to support him. Soon after, things calmed down and seemed normal between Jake and Allyson.

One day, however, Marc was changing a tire in the driveway when Jake stopped by. He went into the house to say hello to Kate. Out of the blue, Jake blurted out, “I’ve met the love of my life.”

“You’re married,” Kate said. “Are you having an affair?” She was shocked and disappointed in her friend.

Then came the kicker. “It’s you,” Jake said.

Kate was speechless. She picked up Ari and ran outside. Jake followed and said goodbye to all of them as if nothing had happened. After he drove away, Kate immediately told Marc about the incident. The next day, the couple called Jake, and Kate told him that even if he wasn’t, she and Marc were happily married. She hung up and cut off all contact with her once-best friend. Although it was painful to lose a friend, as far as Kate was concerned, Jake had crossed a line that signaled the end of the friendship.

“You can’t expect everything from one relationship,” comments Lauree Ostrofsky, founder of Simply Leap, a life coaching and communications company in Washington, DC. “Even if your partner is great, other friends (male and female) can really add to and enrich your life,” she says.

But just as same-sex friendships morph over time—and even the best of them don’t necessarily last forever—recognize that a platonic friendship may turn steamy for one individual or another. Having a solid friendship as a foundation should help in successfully renegotiating the terms of the relationship.