I was cycling home along the river one summer evening three years ago when I first saw Sandy at work. He's an artist, and I stopped to watch what he was doing. He saw me and called me over, we talked and at the end of the night we exchanged phone numbers. The next day he called to ask me to visit him again, and we sat by the Thames chatting until the sun came up.

From that time on, we spent a great deal of time with each other, and he moved in with me soon afterwards. A month or two later, Sandy told me his story. Without much buildup at all, he said, "I've got to tell you that I'm on the sex offenders register."

He began to explain that it had happened at a party a few years earlier. He'd met a girl there and went to bed with her. She'd taken him to a bed that her friend had already occupied. They fell asleep and the next morning, as they were getting into things again, the other girl began to stir and brush against him. Sandy had taken this as some sign of interest and began to touch her as well. She wasn't interested and told him so, and he stopped. But she went to the police the next day. He was arrested and charged with sexual assault. His solicitor advised him that if he pleaded not guilty and they found him guilty, he would face a prison sentence. Out of fear, he told me, he pleaded guilty to one count of sexual assault and was sentenced to two years' probation.

Although Sandy was open about his offence from early on in our relationship, the full implications didn't properly sink in with me until about a year later, when the police came round to check that, as his official live-in partner, I was safe and aware of the situation. It was also when I found out that even they didn't know about the case details – the police knew only that Sandy had been sentenced and couldn't tell me any more.

That was the point when it dawned on me: I had no other source who could tell me what Sandy had done. I guess I have wondered about it from time to time. I'll never know what really happened that night, and I'm well aware that human memory can be fallible. I imagine that some people must wonder at how I trust Sandy's version of events. When I tell people, I see a flicker in their eyes, as if they are trying to make up their minds about him. People empathise with our situation, but you never know what they are really thinking.

Five years on, this is still Sandy's reality – and mine. As a couple, we have learned to adapt to a life where he still has to register his address with the police if he goes anywhere for more than three nights. I'm very aware of the parallels with my own history – I was abused between the ages of 12 to 15, and later found out that the man had abused many other children before me. My case eventually went to trial, but it took more than two years. He was sentenced to two years' rehabilitation and never allowed to be in the company of children again.

I imagine the man who abused me probably tells people he's innocent, too, so I do understand how people can behave in these situations. I'm aware, too, from my own experiences, that you can never really know a person. How does anyone know that their partner is telling the truth about their past? The answer is that none of us does – we have to trust our own experience of an individual. I made my decision to trust Sandy early on, and I still do.

What happened to him doesn't define us, and we don't focus on it all the time. I try to be as open as I can about what's happened, because I think people need to hear these things – our close circle of friends know, and it's not some-thing I feel ashamed about. Everyone has an idea of how good or bad people are, but there are grey areas and, as I've discovered, things are not always that simple.

At least we've been through the worst of it now. We've been told that the police monitoring will end in April and we plan to have a big party to celebrate. I don't expect it will change anything in our relationship, but I'm hoping it will be a new start for both of us.

• Sandy is not his real name.