The Man who knew Infinity was Ramanujan of course, and the book is about his life, and how brilliant he was.
One of the chapters discusses Hardy, the famous British Mathematician who got Ramanujan to England. And while trying to analyze his character, it speculates on whether he was a homosexual, in a very matter-of-fact way. But it also discusses what a big deal homosexuality was then, and how Hardy (if he was in fact one) would have spent his life being extremely careful to avoid his preferences being known.
Another chapter discusses how Ramanujan, being Brahmin, couldn't cross the seven seas, else he would have been ostracized from society on his return back to India, and how in spite of his love for mathematics, and his excitement about a great mathematician like Hardy wanting him in England, he was too scared to violate the principles of his religion.
Being homosexual in Britain is no big deal anymore. Neither is a Brahmin guy crossing the sea a big deal in India (in fact, going abroad is a cool thing now, irrespective of caste).
But it was SUCH A BIG DEAL to people just a 100 years ago, even the really smart people.
Makes me wonder what are the things society would just not accept now, which would be matter of fact in another 100 years.