In the summer of 2000, I could never have imagined becoming a father. I was 34, living in New York City, with a good job in social care, but still in a tiny apartment. I had been with my partner, Pete, for just over three years; we were serious, but we didn’t live together. Becoming a parent was not on my radar.

One August evening, I had finished work late and was hurrying to a dinner reservation I had with Pete. I was rushing towards the turnstile at Union Square station when I noticed a bundle of clothes in a corner. I saw it move and stopped in my tracks. I walked over, peeled back a dark sweatshirt, and saw him: a newborn baby, with the umbilical cord still attached.

I was in shock. I sprinted up to the street and found a payphone to call 911. “I found a baby,” I blurted out. I rushed back to the platform and crouched down next to the baby. I stroked his head to comfort him but he pulled a face. “OK, you don’t like that,” I said. We stared at each other. My heart was racing.

It felt like hours, but it was probably only a few minutes before the police arrived. I had to give a statement, and went home for a large drink. Pete and I talked all night; why would the mother have left the baby, why had she chosen to leave him here, in the centre of gay New York?

After a short period of media interest, life returned to normal, until 12 weeks later, when I was asked to testify at a court hearing as the mother could not be found. To my surprise, the judge asked if I had any interest in adopting the baby. The idea hadn’t even entered my head, but instantly, I desperately wanted to say yes. I told her I needed to talk to my partner but, in my own mind, I had decided that was what I wanted to do.

Pete was furious. We had never talked about starting a family. We were in debt – there were a hundred reasons why bringing a child into our lives did not seem sensible. But I was convinced.

Pete agreed to visit the baby in foster care with me. As soon as I saw him, I took him in my arms. “Remember me?” I said. Pete says when he held the baby, every morsel of resistance instantly evaporated. We left that house united.

We were called back to court on 20 December, and granted custody. “How would you like him for the holidays?” the judge asked. We bought parenting books and read them cover to cover in 24 hours, and I moved into Pete’s flat.

We named him Kevin. Pete had an older brother named Kevin who had died before he was born, and his parents always said he had a guardian angel named Kevin watching over him.

Taking baby Kevin home was incredible but terrifying, as it is for any new parent; but, unlike them, we’d had just a day to prepare. For weeks, we took it in turns to sit up round the clock with him to make sure he was still breathing.

We wanted to make sure Kevin knew he was wanted and loved, so we wrote a story for him about how we became a family. He made us read it over and over, and took it to school.

When Kevin was 11, New York legalised same-sex marriage, and we told Kevin we would like to get married. He said, “Don’t judges marry people?”, and suggested the judge who asked us if we wanted to adopt him. We were delighted when she agreed to do so.

Not everything has been easy. When he was a teenager, he had a lot of questions about his birth mother. He wanted to put up posters in the subway, and we would notice him looking at strangers’ faces to see if they looked like him. He’s made peace with the situation now, though.

Pete’s written a memoir, and we also turned the story we wrote for Kevin into a children’s book and had a short animation made. We want other children to understand there are lots of ways to become a family.

Now, Kevin is an incredible young man and we are tremendously proud of him. He works out of state as a software developer but, fortunately, he is still happy to spend time with his dads.

Even 26 years later, we can’t quite believe that, by some miracle, it was us who were given the privilege of being part of Kevin’s life. How lucky we are.

As told to Heather Main

Do you have an experience to share? Email experience@theguardian.com