Well, Leslyn Heinlein claimed that Laffy experimented with Robert, but that's not a greatly credible claim, as Leslyn had degenerated into a delusional alcoholic with an axe to grind by the time she made those charges.
It has been substantiated that Laffy had an affair with Leslyn herself, though. Heinlein had an open marriage with Leslyn, but it's not clear that she was into that, and the affair with Hubbard may have been out of spite.