Derek shared the timeline of events and why he decided to disclose Katie's cheating at the reunion special during a recent appearance on Married at First SightSeason 1 couple Jamie Otis and Doug Hefner's Hot Marriage Cool Parents podcast.
"A month or two prior [to the reunion] when I first found out Katie had the affair while we were on our one-month anniversary [after 'Decision Day'], we talked about it and I was a little shook that whole day," Derek shared.
"I mean, we were in Nashville and I had to fly back to Virginia for my grandfather's funeral... She stayed [in Nashville], so I had a lot going through my head and I had a lot of time to think about things."
"The following day, we talked and we established, "Alright, let's just try to be friends.' But we also agreed that I wouldn't discuss the affair during the reunion, because she already felt like everyone more or less hated her enough from everything. So I'm taking that into consideration and was like, 'Okay.'"
Then, several days before filming Married at First Sight's tenth-season reunion special, Derek said the pair also met for dinner to discuss what they should and should not talk about on-camera with host Kevin Frazier at the reunion, which aired on Lifetime late last month.
"We were catching up, having a drink or two, making sure everyone is doing well -- because we weren't living together at this point in time," Derek explained on the podcast.
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"Going into the reunion, we wanted to make sure we were on the same page with things and make sure we don't bury anyone, I suppose."
Doug noted he couldn't imagine that Derek set out to call Katie out for on cheating at the reunion, and Derek agreed, saying his initial plan was to remain tight-lipped about the affair even though he and Katie were not on the best of terms after deciding to end their marriage.
However, Derek admitted, "I always kind of had this in the back of my pocket. I was like, 'Okay, you know, I said I wouldn't talk about it, but if you end up being a real twat, or whatever you want to call it, then I have no qualms about calling this out.'"
Derek said his agreement with Katie then essentially went out the window shortly after they met for dinner.
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"Then, literally just a couple of days before the reunion filmed, after the dinner, I get this information from three different cast members -- and I verified it with a couple of other people as well -- that not only did she have an affair just two weeks after we moved in with each other, two weeks after 'Decision Day,' but she [also slept with her ex] just a couple of days after the honeymoon, like the day after our honeymoon," Derek recalled.
"And so at this point in time, I am like, 'What the F?! Okay, I'm sorry, but I just have no sympathy now. I don't have any qualms about calling all of this stuff out.' I was fed up with it! I only have so much patience," Derek confessed.
Derek therefore put Katie on blast at the reunion show.
Katie admitted to sleeping with her ex and having "a physical slip-up" after "Decision Day" on the season's reunion special, but she completely denied having an affair after her honeymoon.
Katie insisted at the reunion Derek's informants were misinformed or misinterpreting things.
Regardless, Doug complimented Derek on his patience and for handling himself with maturity throughout the reunion special.
"Honestly, the way she came off on the reunion, it was almost like she was blaming you for her cheating, which was weird! It was so weird! She was blaming you for her cheating," Doug acknowledged.
Doug was seemingly referring to how Katie had complained she and Derek had no physical connection during their marriage.
At one point during the reunion special, Katie even accused Derek of not making moves on her in the bedroom or initiating intimacy of any kind.
"Yeah," Derek agreed with Doug. "Yeah."
Derek then clarified the timeline under further questioning from Jamie, claiming Katie cheated on him with her ex the day after they returned from their honeymoon and then slept with her ex again two weeks after Derek and Katie had moved into a new apartment together following their decision to remain married on "Decision Day."