A natural harmony between Genesis and Peter is found in the harmony between Peter and Paul. Someone who reads the Bible as a disaparate collection of different people's opinions might object that to import Paul is the wrong way to answer the question. But such an approach does not even require a reconciliation between Peter and Genesis. Copies of the Scriptures were not as plentiful as they are now, Peter was an uneducated man, and someone who holds that position has no reason to be puzzled if he practices historical revisionism or drew from a different source about Lot. Thus, to even ask for reconciliation between Peter and Genesis presupposes the reasonableness of harmony between Peter and Paul (i.e. on the basis of the inspiration of Scripture).
Lot's offer of his daughters was a horrifically wicked action. Any man who does not diligently guard the chastity of his daughter greatly offends a righteous God. A man who would encourage her to be unchaste is worse; but a man who offers not one, but both, of his daughters to not one lone, but a whole town full of men, is a criminal of the first order. Indeed, my own inclination would be to say that Lot's offer of his daughters is more wicked than the men of Sodom. If we do not emphasis the great monstrosity of what Lot did, we do not uphold the moral character of God. The man was vile!
How then is Peter able to hold forth Lot as a righteous man? Because he holds to the same doctrine of righteousness as Paul—alien righteousness through faith in the Messiah. Lot was more righteous than the men of Sodom because he believed in God. This faith is evidenced by his distress at the wickedness of the people around him, as mentioned by Peter; but were we to tally up his righteousness, his distress at the wickedness of the Sodomites would certainly have been far outweighed by his own lack of moral fortitude. Distress at the wickedness of others is hardly a thing of positive merit—but it can be a sign of a heart which faith has begun to soften. The only possible way to consider him righteous, then, is to recognize that his wicked deeds were washed away by the blood of the Messiah. This salvation by faith that he experienced resulted in his disgust at the evil around him, even though his faith remained wavering and imperfect.
If Peter's doctrine is to be separated from Paul's, then why not Genesis also? And to man who minimizes the horror of what Lot suggested, what can I say?—may daughters never be born to him! Justification by faith alone is the only viable way to reconcile the two passages.