The Fool's Gold actor's mother Kay made the revelation in her new book I Amaze Myself! and also admits she ensured James was taken from the house naked following his death.
She said: "On Monday mornings, he and I often said goodbye by making love. But one day, all of a sudden, it just happened.
"I knew that something was wrong, because I didn't hear anything from him. Just nothing. But it was just the best way to go! I made sure he was naked when they took him because I was just so proud to show off my big old Jim McConaughey - and his gift."
Matthew recently revealed he wants to die in the same way his father did.
He said: "I can't think of a better way to go. My dad always said to me, 'Boy, when I get out of here, I bet you I'll be making love to your mother.’
"I hope my own death is way down the line, but when it happens, that's my preferred exit strategy. But let's not rush it, eh?"
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