Mr Lewis, from Trowbridge, died in a car accident on New Year`s Eve on Lamby Way in Cardiff while travelling with his pregnant wife and their three-year-old daughter.
The baby was delivered by C-section at the University of Wales Hospital, Cardiff, three days after the crash, but did not survive.
Cardiff Council, which runs the crematorium, has launched an immediate investigation into how the inappropriate footage - which was shown for almost four minutes before being turned off - had come to replace a recorded tribute to Mr Lewis.
One of the mourners, who wished to remain anonymous, told Wales Online it was "absolutely disgusting".
“Everyone was in shock,"they said. "Simon’s father-in-law was furious, he shouted at the staff present and told them to turn it off.
“It took them nearly four minutes to turn it off, people couldn’t believe what they were seeing."
The funeral-goer said that the priest had handled the situation "extremely well", apologising on behalf of the coucil and telling mourners that in 30 years of doing the job, he had "never seen such filth".
Reverend Lionel Fanthorpe, who led the funeral service, told the news outlet it seemed to be some sort of "electronic incident".
"I remember looking up at the screen and seeing some type of video came on and a very loud noise.
"I look at my congregations instead of the TV screen, but when I heard this noise and members of the congregation moving towards the screen, I realised something was wrong."
He said that members of the family were very distressed, and Mr Lewis`s father-in-law was "desperately upset".
He added: "I was extremely saddened for the family at losing Simon and the baby, and this was not what anybody could have ever imagined or wanted."
A City of Cardiff Council spokesman said the council had sent a written apology to the family and confirmed the screen had since been completely disconnected.
"There were four television screens used to display visual tributes as part of this funeral service," he told Wales Online.
“The television screen which showed the inappropriate content was recently installed, replacing a screen which was broken.
"We are trying to establish if the new screen – which is a SMART television – could have accepted or picked up a broadcast by accident via blue tooth or across a WiFi network.
“The other three screens which aren’t SMART TVs were unaffected. We are clear that it isn’t possible for any member of staff to play or download anything on the computer that links to the screens in the chapel."
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