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Theirs is one of the most rock-solid marriages in Hollywood.

Despite the 16-year age gap and the weeks apart while filming, Ashton Kutcher and Demi Moore are still besotted with each other eight years after they first met.

And perhaps that's just as well, given the steamy romps Ashton took part in for his latest film No Strings Attached.

The adult-rated movie is the ex-Calvin Klein model's hottest yet, full of nudity and flirtatious dialogue.

He plays a man who embarks on a sex-only relationship with a close friend, played by actress-of-the-moment Natalie Portman.

But despite his sultry ­co-star, Ashton says he found ­filming tough going.

"I was shooting so many sex scenes I was getting tired of it," he says. "I never thought that anything related to sex would be something I'd get tired of!

"You're there and you're doing the scene and you're like, 'Do we have to shoot another sex scene?'

"You're always waiting, wondering when the word 'cut' is going to be said. How far are we taking this? Are they going to call cut?

"The director would come back and say, 'I think you need to orgasm sooner'. So your male machismo is like 'No, no, no. It would take me much longer than this'. I'm sure that every actor says it, but it's always slightly more complicated than it is in real life."

Ashton, 32, reveals he had a secret sex scene technique for the movie& which he learned from acting legend Sir Laurence ­Olivier.

"I just start by apologising," he says. "When in doubt, use Sir Laurence. He said something like 'I apologise if I get aroused and I apologise if I do not get aroused' and you have to say it with the accent.

"But there's sort of always that kind of awkward state of like, 'Is this OK? Is that OK?' Then in ­between it's like, 'Let's act like nothing happened', and then you see how good an actor you are."

When it comes to his real life with Demi, Ashton says there is no secret at all to their success.

"At the end of the day, I think it's about the right wife - finding somebody who is the right one," he laughs.

Demi, 48, is clearly the right one for Ashton. She had just turned 40 when they met and he was a rising star of 25 most famous for his model looks and string of starlet lovers.

They wed five years ago but still behave like love-struck teenagers, posting saucy pictures of each other on the internet and Tweeting soppy love messages.

But he admits he does have to work at maintaining their spark.

"I'm constantly reminding myself, no matter how amazing my relationship is, keep working on it. Things can always get better, even when it's going great.

"I think romance sort of coincides with effort. Recently, I was working in another country and I had one day off. So I flew home to see my girl for an hour and then turned around and went back.

"You can fall flat on your face but, as long as you're making a great effort, I think it comes off as romantic. It can be something simple. If you're someone who doesn't cook, you can make a meal. It's anything that has a little bit of vulnerability and requires great effort.

"I really think you have to find a partner that complements you and is somebody that pushes you and is better at some things than you are, so they can push you to improve yourself as a person."

Talking to Ashton, it's clear he and Demi are incredibly well-suited. Even Bruce Willis, Demi's 55-year-old ex - who had three daughters with her during their 13-year marriage - gave his ­blessing.

Ashton says the success of their marriage has been down a bit of give and take.

"If you ­really want to make a relationship work, at some point in time you're going to have to make some sacrifices and do some things that are a little bit uncomfortable," he says.

Uncomfortable was how ­Ashton found himself during the shooting of No Strings Attached. The ­relationship of the two main characters - friends seeing if they can have emotionally-free sex - is not one he thinks would work for him.

"I haven't been fortunate enough to try one of those relationships out," he jokes. "I really think that whoever you're with ultimately needs to be your friend.

"In all the really successful, happy relationships I know of, the people are friends anyway. And I don't know that sex always has to have ­feelings, but I think that friendship always does."

Despite his former ladies' man image - his exes include Lindsay Lohan and the late Brittany ­Murphy - Ashton comes over as something of a feminist.

"I think there's so much that's not said about sex," he says. "There's a lack of education about sex as a point of pleasure for women.

"The male orgasm is right there and readily available to learn about but a female orgasm isn't really talked about in the education system.

"So, to create a movie with a female lead that is empowered by her own sexuality is a really powerful thing."

Ashton is full of praise for co-star Natalie, 29.

"I learned more from Natalie in one day of being on set together than I can ever possibly teach her," he says.

"She may not have done a rom-com before, but she's done so much work on so many different ­levels."

And behind his easy-going manner there is a real workaholic. As well as his acting and helping to bring up the children, he runs a TV production company with his friend Jason Goldberg, making series such as Opportunity Knocks, Beauty and the Geek and The Real Wedding Crashers.

"Most people are in disbelief that I go to the office every day," he says. "When I'm not acting on a movie, I go to work, first thing in the morning.

"I'm at work at eight and I get home at seven at night. I treat my job like a job and I work at it.

"If I ever added up the number of hours I work in an average week and published that, I think people would be shocked."

adamleepotter@sundaymirror.co.uk