Kate Winslet and Leonardo DiCaprio have kissed in the past. They've done it a lot. Winslet and DiCaprio kissing was more-or-less the backbone of 'Titanic,' which was one of the biggest films of the 1990s. When they were cast together again in 'Revolutionary Road' over a decade later, they refused to kiss, or perform any romantic scenes at all. There are a few reasons why, and none of them have anything to do with not liking each other.
The biggest thing that stopped Winslet from kissing DiCaprio is that her husband Sam Mendes was sat in the director's chair for the film, and she didn't feel like she was able to kiss someone she was so famously connected with in the past while her husband was sat in a chair a few feet away. DiCaprio wasn't offended by her decision; he felt that kissing scenes between himself and Winslet would have reminded the audience of 'Titanic' too much, when he'd rather they were focused in on the newer movie. Everyone was happy - except, perhaps, the audience.