In Christopher D. Felker's review of Hoffman Andrew.  _Inventing Mark Twain:
The Lives of Samuel Langhorne Clemens_. he quotes:
"Much later, after his marriage with Livy and on a trip to the Louvre in
Paris, Clemens supposedly "found himself appalled at Titian's "Venus," who
stares frankly back at the viewer while apparently masturbating " (267). "

Isn't that painting in the Uffizi in Florence?

I can recall the moralistic dudgeon with Which Twain writes about it, but
can't remember where that writing is.

Venus on the Move