"It almost seemed like Katie had no intention of following through on staying together [since she cheated] soon after you guys decided to stay together," Doug Hehner noted on a recent episode of his Hot Marriage Cool Parents with Jamie Otis and Doug Hehner podcast on which Derek appeared as a guest.
"Yeah, very much so," agreed Derek, who has claimed Katie cheated on him with her ex-boyfriend two weeks after "Decision Day" was filmed and the MAFS couple had moved into a new apartment together.
"That's why to me, I can honestly say that I was never in love with Katie. My feelings were never allowed to grow to that, but I still felt hurt and betrayed when I found out about it the first time."
The "first time" Derek found out Katie had an affair with her ex-boyfriend was when she revealed it to him during a trip they had taken to Nashville, TN to celebrate the one-month anniversary of their "Decision Day" decisions.
"It was like, 'I'm here actually trying to make this happen but you just dismissed it. Like, what was the point of all of this in the first place?' And that's why I was mainly hurt, was the betrayal," Derek explained.
Derek said a couple of days before the reunion special filmed several months later, he also heard from three different cast members that Katie had allegedly slept with her ex-boyfriend the day after they returned from their honeymoon in Panama.
POLL: DO YOU FEEL KATIE HAD NO INTENTION TO STAY WITH DEREK AFTER DECISON DAY?
Derek said he verified the information with a couple other people as well.
Katie admitted to sleeping with her ex and having "a physical slip-up" after "Decision Day" on the season's reunion special, which aired in late April, but she completely denied having an affair after her honeymoon.
Regardless, it seemed like Katie wasn't satisfied during her marriage to Derek, as she often complained about their lack of physical intimacy and Derek's boy-like personality and unrealistic dreams.
"Why did you guys stay together on Decision Day?" asked Doug, who appeared on Married at First Sight's first season with his wife Jamie. "Was that a conversation you had going into 'Decision Day,' that you wanted to stick it out?"
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"Yeah," Derek replied. "The biggest thing for us is that we wanted to try it without the cameras to see if it was just the cameras that added more pressure and more stress to things."
"And, you know, granted it also didn't help that both of our leases expired and all of our roommates had found different situations," Derek added.
"They all had to move too, so it was like, 'Okay, let's try this out without the cameras while also living with each other and solve both of these problems.' Those were the two biggest things."
Derek, however, said he still had hope things could change for the better in his relationship once he and Katie were living together.
"The last thing I wanted to do was go through this process and give up halfway without knowing for sure at the end. That's the one thing I told myself I didn't want to do," Derek said.
Doug acknowledged on the podcast the way Katie had treated Derek was "really sh-tty."
Doug also said it seemed like Katie constantly judged him when Derek never even gave her a hard time about her lingering feelings for her ex-boyfriend, who had professed his love and tried to reconcile with Katie before she walked down the aisle on Married at First Sight.
"The whole thing with the ex-boyfriend -- not knowing if she was in love and the breakup [without closure] -- you never held that over her head throughout the experiment. It was like you were willing to look past some of her stuff but she wasn't willing to look past the things that came up about you," Doug explained.
"Yeah, it's funny you say that because I actually never really noticed that," Derek said in response. "You're right. I never held those things against her."
During his podcast appearance, Derek also explained why he decided to reveal Katie cheated on him and had an affair with her ex-boyfriend despite initially agreeing not to discuss it on Season 10's reunion special.
"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 said.
"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.
"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.
"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.
"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.