I never noticed any of those when watching either. Which brings me to the conclusion that it wasn't used to encourage kids or anything, but probably for the amusement of the Disney animators, which they're entitled to do. Especially in the case of The Lion King, where the word isn't 'SEX' but 'SFX' the abbreviated name of the animation group. Most of these theories aren't true, although the Rescuers one is seeing as it's a picture.
I'd heard about these from Snopes, and it's useful in what it says. Here's the page where the theories are mentioned, and you can click on the links for more information:
http://www.snopes.com/disney/films/films.asp