Asked if he feels threatened by her beauty, she jumps in: 'No, because he sees me in the morning.'
He says: 'I believe our connection is deeper. Must be confident, mah.'
They got married at the five-star hotel, Uma Ubud, in Bali in 2006, after two years of dating.
They say they have many similarities. They are socially and environmentally conscious, often participating in fund- raising events. He is organising a charity swim in January.
They are both vegetarians and greenies.
Their new home, a three-storey, six- bedroom house in Bukit Timah, is designed to minimise energy use.
Water is warmed with solar power and the wood used is from sustainable sources. The house, which has a 20m pool, will be finished by January.
He is also planning to start what he calls a 'climate change fund', which makes investments that have a positive impact on the environment.
Their green causes can be traced to Buddhism, which they say has brought them closer.
But they admit that they have different parenting styles. She is patient and relaxed, while he stresses discipline.
They have disagreed on how to raise Tyrone, who is studying in Australia.
Koh says, with a hint of exasperation: 'It was all the normal things. Don't want to sleep, don't want to shower, don't want to get off the computer, don't want to turn off the TV. For me, it is very simple. If this is your bedtime, this is your bedtime. Follow it.'
He adds: 'My way is more traditional Chinese style. My mother said this, my father said this, so this is the way. Everything ran like clockwork.'
He says he could have 'gone completely haywire' without discipline, adding: 'That was how I achieved my successes.'
She says, with a laugh: 'Then he realised not everyone is the same.'
They have found a middle ground for their parenting decisions but their differences can be traced to their upbringing.
Born to an Australian mother and an Indonesian father, Hutagalung grew up quickly.
She started modelling at the age of 12 and went to Tokyo shortly after on her own. Later, she became one of MTV's most popular VJs and mixed with the rich and glamorous at parties.
While his future wife was painting the town red, the young Koh led a sheltered life that revolved around swimming. Up till he was 16, his bedtime was 9.30pm.
He is the second of three boys of a retired shipping director and a housewife.
All three boys were national swimmers. His younger brother, Gerald, represented Singapore at the Atlanta Olympics in 1996. Now all three are in banking.
Terry, 39, is at Barclays Capital while Gerald, 31, is at investment management firm Pimco.
Swimming was Koh's life. He woke up to go for 5.30am training session, went to school, went home for a nap, did his homework and went back for an evening swim.
The determined child drew up timetables for his revision and tacked his target times for swimming on his bedroom wall.
He says: 'In 1988, I went to the Olympics. I was 15 years old. I had broken so many national records, my academic results were good. I felt untouchable.'
The girls loved him too. His mother had to change the home telephone number to fend off his adoring fans.
After scoring six A1s in his O levels, he went to the University of Southern California and graduated with a degree in electrical engineering.
He kept up with his swimming and took part in the Olympics, the South-East Asian Games and the Asian Games.
He was later awarded a Rhodes Scholarship to do his master's in finance and economics at Oxford University. He chose a finance subject after Terry, his role model, had gone into banking.
At Oxford, he tried his best to stay fit but the cold weather and lack of sports infrastructure proved a challenge. There was no Olympic-sized pool.
He came back to Singapore in 1997 and swam for another year or so. In 1998, at the age of 25, he retired from competitive swimming.
He says: 'There was a bit of sadness. I miss competing. As swimmers, we know ourselves quite well. It comes to a point where even if you want to do it, it is gone.'
Fit and fab
But he still keeps a rigorous fitness regime. He runs, swims or cycles three to five times a week and takes part in triathlons.
'If he does not exercise, he gets mang zang,' his wife says, using the Cantonese phrase for frustrated. She admits that she does not exercise with him because she cannot keep up.
He is still a passionate advocate of sports. In 2001, he set up SPORTinc, a sports marketing firm that manages local athletes and organises sporting events.
Now, the company concentrates on providing swimming and other fitness programmes for adults.
When you ask his friends and family about him, the words 'driven', 'determined' and 'disciplined' come up often.
Koh's friend of 30 years, spa owner Tony Hoon, 44, says Koh is 'very clear about what he wants'.
'When I encourage people to stay determined, the name Desmond Koh always pops up as an example.'
On his friend's marriage, Mr Hoon says: 'He is very in love with her. To start with, Desmond is a very loving person. But when he is with Nadya, you can see the sparkle in his eyes.'
Koh's brother, Gerald, says: 'I look up to him a lot. I am very fortunate to have him as a brother. He has taken me under his wing, whether it is swimming, studies or career, he has always given me good advice.'
Now the good friend and brother wants more kids.
'But I have to be concerned with Nadya's health, that is still No. 1. She is not getting any younger.'
On Nyla, his youngest, he says: 'She has me completely wrapped around her finger.'
HE SAYS
'(I knew I wanted to marry Nadya because) she fed me. She took care of me. The incident that I remember was the time I fell ill with tonsillitis. It was so bad, I lost so much weight. I could not swallow. She was there for me and I felt very comfortable. Also, we are headed for the same direction and we share a lot of values and beliefs'
SHE SAYS
'He kept asking me for a date, I kept saying no. But we hung out as friends. After a while, I began to understand who he is as a person, and the values he has. The kindness and sincerity really jump out at me. He is a gentleman, he still goes out his way to open the car door for me. He is old school'
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