In the polyamorous world, there is a special term for the third person in a relationship. She (and it is usually a she) is called a "unicorn". This is a joke, obviously. Based on the fact that she is rare, beautiful and... virtually impossible to track down.

My partner, Ryan, and I are non-monogamous. When we first opened things up two years ago, we both hooked up with people separately, but we were also eager to engage in our first threesome. This turned out to be a challenge. Finding an emotionally mature, sexually adventurous, bisexual woman who is also our type – a feminine Girl Next Door – is hard to do. A year passed with no progress on the threesome front.

Then, our old friend Kara reappeared in our lives, bouncing up to me at a party. I didn’t even recognise her at first; we hadn’t seen her since she broke up with her boyfriend a year before. With her strawberry-blonde hair and pale, freckled skin, she was the very definition of the Girl Next Door.

“How are the boys treating you?” Ryan asked her while I was across the room. “Fine,” she said, rolling her eyes. “But the girls — I’ve had a lot of fun with girls.” And that’s when he remembered: Kara is queer.

“So, what do you think of my girlfriend?” Ryan ventured. Kara put her finger to her lips, smiled enigmatically, and spun away to dance. Ryan immediately found me to relay this intriguing interaction.

An hour later, on the dance floor, I felt female arms wrap around my waist. It felt too good to be true — like a skittish cat had finally crawled into my lap. I didn’t turn around, but relaxed into Kara's hold. Finally, she spun me around and entwined her fingers in mine. I looked right into her big, blue eyes and she stared back, taunting me. I wanted to kiss her, but I couldn’t go through with it.

The next day, she sent us both a Facebook message. “I had such a happy time seeing and dancing with you two yesterday! Let’s not be strangers!” Ryan and I practically squealed when we saw the message. Within the week, we invited Kara out to see our friends play a gig in Brooklyn. By the end of the night, we were all making out, right in front of the stage — and then running out the door, holding hands, to catch a cab to our flat.

As soon as we got in the door, Kara dragged me to the bedroom and crawled on top of me, giggling. Ryan joined in and all of us were together, exploring one another and marvelling at what we had found.

After that one heady night, Ryan and I started inviting Kara out with us every couple of weeks. We went to parties, had picnics, or cooked dinner at our place. Everything felt perfect. Ryan and I were so thrilled that such an amazing creature would allow us both to have sex with her, we didn’t really notice how she always went for me first.

My friends thought it must be complicated, having a third. But at the time, it was easy. Kara’s a free spirit and was remarkably relaxed about our arrangement. She respected that Ryan and I put each other first. And we were loose with her, too. When we took her to a sex party, we were proud of how easily she met and hooked up with a guy. That’s our girl — the cutest one at the party, we congratulated ourselves.

As the months passed, the three of us became closer. We spent a weekend at a music festival. Kara introduced us to her sister and during one long night, she opened up to us about her grief over losing a family member to murder. At some point, Ryan and I started calling Kara our girlfriend. The day Ryan proposed to me, Kara was there. I told her she should be in our wedding party.

At the end of the summer, I tentatively told Kara that I loved her. She said she loved me, too.

I think that’s when things started to get complicated. I realise now that, while we weren’t exclusive, we also hadn’t clearly communicated our expectations. On the one hand, I didn’t expect this to last forever; I was braced for the day when Kara would come to us and say, “I met someone special,” and I promised myself I would have the grace to let her move on to the next phase. But on the other hand, we’d said the "L" word. We were incredibly close. A sense of ownership had started to creep into my mind.

The group dynamics were off, too. When I went home early one night from a party, and Ryan and Kara didn’t get home until eight in the morning, I fled the apartment and refused to take their calls because I felt left out. But Ryan was usually the one who felt left out, when Kara and I would gang up on him about a feminist issue — or when she and I would have sex, just the two of us.