I am not saying that AL is right, I’m saying that Lownie will have thought about the ethical considerations at length, which have to be balanced with accuracy, context and leaving behind a fair historical record.
Agree with Basiliskstare that AL is not prone to including details for salacious reasons. He will have included this information I imagine because of its relevancy to the rest of the contents of the book.
If a biographer commits to the job seriously and is writing objectively in good faith, they can’t always decide to leave out significant parts of a subject’s life for privacy reasons if it’s highly relevant to the rest, not least because the full truth has a way of emerging eventually, even if it’s decades later.
As long as the author is prepared to take full accountability for what they have written of course, which AL has already said he is fully prepared to do.
Lastly, as Basiliskstare said, I think it’s fairer to judge the author on the basis of the balance throughout the entire book and not just one part of it. I haven’t read the whole thing yet but this article by a BBC journalist seems to be a very fair summary overall and backs the view of Basiliskstare that including this specific but very private alleged information does help to provide a more nuanced picture of PA without excusing his many, appalling wrongdoings.
https://www.bbc.com/news/articles/c24z1l090dqo