> Note his forceful denials of an affair at the end, which we now know to be false.

Do we? Do we know that an affair (as opposed to an unconsummated infatuation) existed at the time of the letter, as opposed to later? A quick googling indicates that Miss Ternan may or may not have returned his affections.

Despite what various public figures would have one believe, it is possible (if not terribly probable) to resist temptation…

Citation for the affair? Wikipedia gives only weak evidence for them living together after the separation. https://en.wikipedia.org/wiki/Charles_Dickens#Middle_years

And if you click the "Ellen Ternan" link there you will see that they lived together for 10 years and are even suspected of having a child (who died early).

Yeah, but he wrote this before he and his wife separated. Living with Ellen Ternan for ten years after that isn't good evidence that they had an affair before it.

There was a recent biography of Dickens by Claire Tomalin that went into some detail, or online I found this: http://www.telegraph.co.uk/culture/tvandradio/3554534/The-se...

Reputation damage control. Same with telling the wife to stay with him even though neither of them saw any benefit in it, the children were already cared for by their aunt and Dickens was apparently "cared for" by a lover.