Hector Penate said that he quickly fell "in love" with Southwick-King after they met in 2007 and that they started sleeping together two and a half weeks later, according to the In Touch story.
"Larry was in New Hampshire and I was training the kids," Penate said in the interview. "Then she made a couple of hints to me that she was ready to go. We were in the car and we just started kissing."
He also told the magazine that he went on vacation to Mexico with Southwick-King and her two sons with King, Chance and Cannon, and that the two "had sex in Larry's bed a lot."
He claimed she also paid his rent, bought him extravagant gifts and wanted to have a baby with him through in-vitro fertilization.
"She wanted to have a little girl with me," he said, according to the report.
Penate also claimed that Larry knew about the affair but didn't care because he was having an affair with Southwick-King's sister, Shannon Engemann.
"I still think to this day that Larry is in love with Shawn's sister," he added.
King and Southwick-King filed for divorce on April 14. Both cited "irreconcilable differences" as the reason for the split.
Southwick-King has denied the affair with Penante, and Engemann has denied having an affair with King.
Penate's interview with In Touch Weekly is now on newsstands, and his interview with ET airs tonight.