They are both happily married, but Good Will Hunting duo Matt Damon and Ben Affleck are each having illicit love affairs - in spoof YouTube videos, that is.

The pair star in two separate - rather risqué - clips that have become online phenomena after being convinced to feature in sparring skits by American comedian Sarah Silverman and her TV host show boyfriend Jimmy Kimmel.

Matt Damon first became part of the elaborate plan after agreeing to a duet last year with Silverman, in which she revealed to her beau Kimmel: "I'm F***ing Matt Damon."

Then last week, Kimmel released a star-studded response, called: "I'm F***ing Ben Affleck," and now each video has been viewed more than 8million times.

Scroll down for more and to watch Matt Damon with Sarah Silverman here...

Spoof: Matt Damon teamed with comedian Sarah Silverman for her video 'I'm F***ing Matt Damon'

It was part of a long-running in-joke that began three years ago with Kimmel, who was then interviewing minor stars, closing his show Jimmy Kimmel Live with a fake apology to Matt Damon, saying "we ran out of time".

The line stuck, and he continued to use it to wrap up each show.

In September 2006, a much-amused Damon finally agreed to appear as a guest, but Kimmel gave him such a long introduction, he did, in fact, run out of time.

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Fake lovers: The video was shown to Silverman's TV host boyfriend Jimmy Kimmel

Joke: The clip was part of a long-running in-joke between Damon and Silverman's boyfriend Kimmel

The video with Silverman was planned as a "payback" and Damon follows Silverman's expletive lyric with: "On the bed/On the floor/On a towel by the door/In the tub/In the car/Up against the mini-bar."

Kimmel called on Damon's best friend and Good Will Hunting cohort Ben Affleck and released his revenge video last week, in which the pair get up close and personal and declare their affection for one another.

Scroll down for more and to watch Ben Affleck with Jimmy Kimmel here...

Reply: Jimmy Kimmel responded to the video with his own clip, 'I'm F***ing Ben Affleck'

The host also managed to convince a string of Hollywood stars including Cameron Diaz, Robin Williams, Ashlee Simpson and Don Cheadle to appear in the clip as a We Are The World inspired gospel.

Brad Pitt also makes a cameo as a FedEx delivery guy as does Harrison Ford, sending an air kiss to the couple before driving off in his car, which features a "Honk if you're f***ing Ben Affleck" bumper sticker.

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Strange bedfellows: The video has gone on to become a YouTube hit

Cameo: Kimmel and Affleck's video also included a guest appearance by Brad Pitt

A-list: Harrison Ford also stars in the clip. He later reveals a sticker on his car's bumper bar that reads 'Honk if you're f***ing Ben Affleck'

"They called and immediately I was like, 'Yes, it should be this idea...' and they had the same idea more or less already. I have a lot of respect for the group over there – they're really good comedians – so I just committed to it."

Damon's spokesperson also said: "He had so much fun."

Star-studded: The likes of Robin Williams, Cameron Diaz, and Don Cheadle feature in the We Are The World-inspired choir