Bama's correct - it's a crock.
When you have a historical figure who thousands of books have been written about, the urge to "discover" new facts becomes essential - even if you have to make them up. This idiotic rumor has been widely promoted by a so-called historian by the name of Jean Baker. I've heard her claim the same about at least two other CW figures.
So far I've heard Lincoln has syphillis, and a hundred other diseases, he never loved Mary Todd, he was gay....blah, blah, blah. The next author that wants to sell crap books will claim he was a transvestite who was having an affair with Tad's pony.
Was he REALLY!...I hadn't heard that, wait till I tell the boys!