The interior designer told his 'I'm A Celebrity... Get Me Out Of Here!' campmates that he enjoyed the teenage romance, but always knew something "wasn't right" but has stayed in touch with the unnamed woman.
He said: I don't know if I fell for her, I was 16-and-a-half it was at the tail end of school, it didn't feel right, it didn't work for various reasons because I'm gay, but I slept with her.
"It was good at the time and she always tells me she knew, maybe I wasn't very good! But we stay in touch, she's married with children now.
"I think I knew at the time I was with her but at 16 I was a quite a naive kid so I wasn't sure and then you get to 18 and I went off to university and I veered towards boys."
Justin spoke more about his younger days, saying he felt "lucky" because of how accepting people were of his sexuality.
He added: "I was a lucky kid some kids have a tough time but I just got lucky, even when I was struggling in my early teens and thinking something was different and knew I was likely to go down a different path but I had a really accession through childhood really supportive family and friends who have remained that way."
Meanwhile, openly lesbian star Sam Fox says she believes she and partner Myra Stratton will be together forever.
She said: "I knew as soon as I saw her that I felt something, it was as soon as she opened the door to me.
"I couldn't live a lie, a lot of celebrities are but never admit that they are gay and I thought I want to be with her forever, love conquers all and when you find love it's important to keep it. I know I'll be with Myra until the day I die."