Islamic Sate fighter Yago Riedijk, 27, is being held in a Kurdish-run detention centre in northern Syria.
His wife, who is 19 and has had three children, is reportedly living in a refugee camp near the Iraqi border.
Ms Begum was 15 when she ran away from her home in east London to join IS in 2015.
They married before she had turned 16 and Riedijk was 23.
The pair are said to have fled Baghouz, the group’s last foothold in eastern Syria, as its territory collapsed in recent months.
Riedijk, who is said to have rejected IS, was found in the detention centre by the BBC.
Asked if he thought marrying a girl of that age was acceptable, he said: “To be honest, when my friend came and said there was a girl who was interested in marriage, I wasn’t that interested because of her age, but I accepted the offer anyway.
“We sat down and she seemed in a good state of mind. It was her own choice, she was the one who asked to look for a partner for her.
“Then I was invited and yeah, she was very young and it might have been better for her to wait a bit. But she didn’t, she chose to get married and I chose to marry her.”
Ms Begum, who said she wants to return to the UK, has been stripped of her British citizenship.