Mr Knightley fell in love with Emma when she was 13 years old. He was 30 at the time. In most jurisdictions today, that is pedophilia; but we overlook that, because it was no such thing in Jane Austen's time, when the legal age for female marriage was 12. This was only pragmatic; a girl might become pregnant at that age; there had to be a path to respectability, and avoidance of the taint of illegitimacy.
What Jane almost certainly didn't know, but we do today, is that Emma would not reach her sexual peak until her early 30's, when Mr Knightley would be fiftyish, and quite possibly suffering early-stage age-related impotence. He would certainly be 30 years past his sexual prime.
Could Emma have been tempted to dally, then, with some young stud? Would Mr Knightley have consented to look the other way, knowing it was only a dalliance, and that he was secure in Emma's love?