The claim isn't that her attraction to him is only physical—he doesn't get conjugal visits anyway. It's that she only wants to marry him because she literally wants his body, so she can charge people to look at it after he dies.
Would people pay to see it? I have no doubt they would. See, e.g., "Battle Over Custody of Leg Escalates," Lowering the Bar (Oct. 1, 2007) (noting that guy who found a leg in a barbecue smoker he bought at an auction was making "good money" charging $3 a look before leg's owner objected).
Anyway, this claim is being made by author Daniel Simone, who has written a book called The Retrial of Charles Manson that may or may not be published in the near future. The claim does not appear in that book, apparently, but Simone told the Huffington Post that he got the info from his co-author.
In fact, as my source for this pointed out (thanks, James), the evidence supporting this claim does not seem especially reliable (and would certainly not be admissible) because it is not just hearsay, it is multiple hearsay:
In an interview with The Huffington Post, Simone said the information originated with Frank Reichard, an inmate who "has a cell" near Manson's at California's Corcoran State Prison.
Simone collaborated on his book with writer Heidi Jordan Ley, who Simone says communicated with Reichard via letter. In a letter that Reichard sent to Ley, which Simone provided to HuffPost, the inmate said he overheard a conversation in the prison's visiting area between Manson's fiancee, 27-year-old Afton Elaine Burton, and her alleged co-conspirator, Craig Hammond [in which they discussed the plot].
So let's say Manson could sue his quasi-fiancee for something to which this would be relevant. Conspiracy to commit marital fraud in order to obtain a corpse for display purposes, how about that? If Manson called Simone to testify, Simone would be offering triple hearsay: he heard it from Ley, who heard it from Reichard, who heard the defendants say it. Multiple hearsay is not necessarily inadmissible, but it usually is because the party offering it has to have some exception for every link in the chain. As many of you know, there are many exceptions for statements that the Rulemakers consider more likely to be reliable. "Party heard it from an inmate" isn't one of those, unfortunately, but maybe each person was making an "excited utterance" (Fed. R. Evid. 803(2)). Not sure that applies to letters at all, though, except maybe very short ones. Nothing else seems to apply.
I keep hoping for an opportunity to apply Rule 803(13), but you just don't come across "statements about personal or family history contained in a family record such as an ... engraving on an urn or burial marker" all that often. At least not in my line of work.
Simone claims that, hearsay or not, this evidence was enough to get Charlie to cancel the wedding. Burton denies that and attributes the non-marriage to "an unexpected interruption in logistics," which is a pretty impressive excuse for someone who, to my knowledge, has no legal training whatsoever.