Ray J Says Kim Kardashian Slept With Him While She Was Married To Her First Husband

Ray J is speaking out about his ex, Kim Kardashian ... and the fact that she allegedly had an affair on her first husband, Damon Thomas, with him!

In his new book, "Death of the Cheating Man," Ray J says "KK" pursued him and slept with him. He adds:

"She let me know she wanted to get with me. She left her husband for me as soon as we started having sex."

Brandy's little brother wrote the book with Maxwell Billieon.

In the tell-all, the rapper also talks about Kim's big spending habits and her skills in the sack. He adds, "I had no respect for her."

Yikes! Wonder if Kim's ex Kris Humphries will be picking up a copy?