The singer, who was romantically linked to Whitney Houston before her death, has spoken out about his past relationships in a book titled Death of the Cheating Man.
Ray J speaks of a woman with the same initials as the star he previously dated in the book, written with publisher Maxwell Billieon.
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"For years KK and I had a great sex life," he wrote. "There was more to our relationship, but the majority of it was about our wild and extreme sexual chemistry.
"She was a straight freak who was down to do whatever, whenever and that seriously hypnotised me... We were like animals; sexually free to try anything, and we did."
An insider told Life & Style magazine that Kardashian is "furious" about the allegations, adding: "There's so much that has gone wrong for her this year - he's just another person trying to humiliate her."
Kardashian's public profile shot up when a sex tape featuring her and Ray J - believed to have been recorded in 2003 - surfaced in 2007.
She married music producer Thomas in 2000, with their divorce finalised in 2004. She has denied cheating on him.
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Ray J, the brother of singer Brandy, added of 'KK': "To be honest, the whole thing started off wrong. We'd known each other for a while before we dated and there was a mutual attraction, but she was married.
"She let me know she wanted to get with me. She left her husband for me as soon as we started having sex and things between us got intense really fast. After that, I felt obligated to be with her."
Kardashian is currently divorcing her second husband Kris Humphries. She filed for divorce last October after just 72 days of marriage to the basketball player.