The Oscar-winning actress was so disturbed by the relaxed attitude of her husband Sam Mendes - who she is now separated from - to the raunchy scenes in 2008 film 'Revolutionary Road' it made her question her marriage.
Leonardo told Reveal magazine: "It was certainly a difficult time. She was really worried that Sam wasn't bothered by the fact that his wife was making out with another guy right in front of her. I told her it was only acting, but she kept on saying, 'This is really weird.'
"When Sam started telling her exactly how to have sex with me she didn't like it at all. She was freaking out because she was supposed to be having sex with her best friend - me - while her husband directed. But I didn't find it weird at all. It didn't bother me, because that's part of acting."
Kate and Sam - who married in secret in the Caribbean in May 2003 - split in March by "mutual agreement" and Leonardo thinks Hollywood was responsible for the death of their marriage.
The 35-year-old actor - who has been close friends with Kate since they starred in 'Titanic' together - said: "She loved being a wife. Some people are meant to be married and I think Kate's one of them, but Hollywood and this movie business can be a killer for marriages."
Shortly after their split it was reported Kate and Sam had a trial separation in 2009 after falling out while working together on 'Revolutionary Road'.
A source said: "Working and living together was incredibly intense and basically overwhelmed their relationship.
"The pair have a very different work ethos - Sam wanted to switch off after a long day and Kate liked to analyse every nuance of her performance. It was just too much."
The couple raise two children together, son Joe, six, and Kate's daughter Mia, nine, from her marriage to Jim Threapleton.