In Jack's timeline, Stephen had only recently married Diana, but not being very attached to the notion of monogamy himself, he felt no particular disapproval of Stephen taking up with a mistress. It was only natural - Diana was far away, and Sarah was a beautiful young woman; any man would do the same (or at least, Jack would). His raised eyebrows indicated his suspicion that something had gone amiss between Stephen and Sarah, though he knew better than to ask directly. No doubt Stephen would tell him in good time.

"Well, then, perhaps it is for want of someone to practice with," Jack replied. "I am sorely out of practice myself. I would like to try that Bach partita we played that one time - do you remember? It was very fine."