Islamic State 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.
In Syria yesterday columns of black smoke billowed from the last small piece of territory held by Islamic State militants as US-backed fighters pounded the area with artillery fire and occasional airstrikes.
Commanders of the Kurdish-led fighters known as the Syrian Democratic Forces (SDF) said fierce clashes were taking place as they advance toward the last speck of land controlled by the Islamic State group. The militants fought back with snipers, suicide bombs and booby traps.
Fires still smouldered from the area and ammunition exploded time and again a day after an airstrike hit a building, setting off a huge blast.
“It must be a main weapons depot,” said Sefqan, an SDF commander using only his nom de guerre.
The US-backed forces resumed an offensive to recapture the tiny area in the village of Baghouz in eastern Syria on Friday night, after a two-week pause to allow for the evacuation of civilians from the area. Retaking the sliver of land would be a milestone in the devastating four-year campaign to end IS’s self-proclaimed Islamic caliphate that once straddled vast territory across Syria and Iraq.
However, the group continues to be a threat, with sleeper cells in scattered desert pockets along the porous border between the two countries.
A few hundred IS militants, many of them believed to be foreign fighters, remain holed up inside Baghouz, with an unknown number of civilians.
Mustafa Bali, the SDF spokesman, said coalition airstrikes destroyed several car bombs during the past two days of battle in Baghouz. In a tweet, he said three car bombs that were trying to hit SDF positions were destroyed.