Interesting that Christ uses Noah and Lot as examples for the Last Days.
Interestingly enough, Christ does not name even one specific sin, other than they were all busy eating, drinking, buying, selling, marrying and giving in marriage (their children).
Luke 17 and Matthew 24 if memory serves (someone correct me if necessary).
Luke 21 has some interesting things - including Luke 21:34-36.
maybe the sin was that they were living their life, doing what they wanted and totally ignoring God. That’s my first take although certainly a lot more could be said about the verse.