McDermott, 52, revealed that it occurred early on in their relationship.
"When T and I first got together, I had Jack every other weekend," the Slasher star explained of his now 20-year-old son, whom he shares with ex-wife Mary Jo Eustace.
"He couldn't sleep one night and — we were making a lot of noise — and ... I just happened to be looking that way, towards the door," he said to co-hosts Adam Hunter and Nicky Paris as well as Monday's guest host Jay Mohr. "I saw the doorknob turn — I didn't hear it, I saw it turn — and I was like, 'Oh s—,' and then a crack of light."
He said he was able to think on his feet to come up with an excuse about the noises Jack had just heard.
"I jumped off and sat on the edge of the bed," he recalled. "He said, 'Dad, is everything OK?' and I said, 'Yeah, I just got a migraine.'"
In addition to Jack, McDermott is also dad to five children with Spelling: Liam, 12, Stella, 11, Hattie, 7, Finn, 6, and Beau, 2. He said he and Spelling haven't struggled in terms of keeping their sex drive alive.
"We go every day," he said. "It never stops... my wife is so f—ing sexy."
"She's sexy as hell. I look at her, and she just oozes sexuality," he added.
Although his attraction to Spelling, 46, has never been an issue, he opened up on the podcast earlier this month about the affair he had five years ago when he cheated on her. He explained that he didn't have a good father figure to look to as an example while growing up, which affected how he learned to "be a man" on his own.
"I really, really wish that I had been given the knowledge of 'think with your head, not your penis,' because I did a lot of thinking with my penis," he said on the debut episode. He added that his father didn't really give him "any advice" and that he learned "how to be a man" on his own.
"I got three older sisters ... lost my mother when I was 15, she passed away, and then Dad wasn't around," the Chopped Canada host, crediting Jack for teaching him a lot.
"Having a kid the first time was what really taught me how to be a parent," he said. "As far as being a father, I sort of learned as I went along. ... I made a lot of mistakes, but I learned a lot. Six kids later, I'm still making mistakes."
McDermott and Spelling married in 2006, with reports surfacing in 2013 that McDermott cheated on the Beverly Hills, 90210 star with then 28-year-old Emily Goodhand while on location for Chopped Canada. The aftermath of the scandal was documented for two seasons on the Lifetime series True Tori.
"It's so not about the person you're with," McDermott said on the podcast. "For me, it wasn't about T. It was about inadequacies in myself, I felt so bad about myself, I felt like a piece of s—, I needed to do that to feel some kind of power. It was horrible and I'm an asshole, piece of s— for doing it, [but now] our relationship is better than ever. That was a rough Christmas, we came out the other end."