I often read forums dedicated to fans of old school actors. There are Jean Harlow forums, Carole Lombard forums, etc. Some of the fans have stories of information they got from twice removed sources at some point in their lives - growing up, or in an era before everyone who knew those stars died themselves. (Like one fan had a chance to hang out with Robert Stack at one period of her life and she tried to pump him for info about Carole Lombard.) Or they get stories from their parents who knew someone or met someone. So one fan, a Carole Lombard fan, says when she was growing up her mother was in security and was doing personal security at a hotel for George Raft, an old time movie actor who was an old boyfriend and co-star of Carole Lombard, and was crazy for her. The daughter was hanging out with her mother one day with Raft at the hotel and was pumping him for everything he could remember. He hated Clark Gable (Raft did). The fan was asking him about Carole's death and what happened, why did she take a plane, etc. Raft said it was because Gable was a "whoremaster" who was driving Carole crazy by "fucking Lana Turner and that broad from Gone with the Wind."
I can't remember all the specifics that put these fans in proximity at one point with old time actors or people who knew the stars, but it was usually undramatic and the old time people were happy to talk. I believe myself that Vivien Leigh and Clark Gable fucked, because they were both whores, and because you can't read about Gone with the Wind without reading that Vivien hated kissing him because of his terrible denture breath, yet when you read bios of women it is known he slept with, such as Joan Crawford, his wives, etc., nobody is mentioning his breath. It's a very Vivien-specific problem, apparently. Then "Gable" wrote an essay about working with Vivien, to go along with the release of the movie, explaining in many many paragraphs that, basically, they never fucked because he loved Carole and she loved Olivier. (He didn't say fuck, but you get the point.) It seems an odd article for a fan magazine to want to publish.
Finally, when I was growing up, it was right before VCRS so I only saw old movies at college showings or on television. But I read old movie books and saw a thousand stills from GWTW. It's customary for actors to "pose" for the still camera on set a la the scene they were shooting that day. Well, the stills of Gable and Vivien Leigh had so much kissing, I thought the movie was going to have a lot of kissing. There's a still of them locked in a full body embrace with a huge kiss while she's wearing that red dress Rhett makes Scarlet wear. There's a still of him comforting Scarlet when she wakes from a nightmare and then another full body embrace and huge kiss. Basically every scene they were in together, there's a movie still of them locking lips in a pretty major way. And finally I see the movie and none of these kisses are in the movie. Of course he's not kissing her when she's in the red dress - the characters are estranged. He does not kiss her when he comforts her after her nightmare. He does not kiss her in all kinds of scenes. The two movie kisses between them I recall are when he kisses her when she's a widow, and when he kisses her while they're fleeing Atlanta. But the movie stills are just nonstop kissing. It's a little weird considering her reported issues with his breath and they're kissing more than the movie.
When it comes to old movie stars, and not just these two, I have come to believe that when a biography or a story suddenly stops in the middle to announce that two people did NOT sleep together and here are the reasons why, then they probably did.