Actually, as a legal point, calling someone a homosexual or saying that
they have experimented in such things, despite our enlightened age, is in
fact slanderous in all states in which homosexuality is illegal. If I
called a man a rapist and a murderer without proof, he (or his estate)
could sue me for slander and libel. The same is true in any state that
has "Crime against Nature" laws on its books (I think this is virtually
all states, except Hawaii and California, although I'm not sure).
Often, these laws are not active, so no suits occur, but legally they can.
I'm not sure if the Twain estate could sue if they wanted to, considering
how long dead he is. I haven't seen the C-Span interview, so I don't know
if there is cause to sue either.