Matthew 24:36-40 should actually be, and makes more sense as (I've color coded related portions to show how the thoughts are linked together logically):
"“Concerning that day and hour nobody knows,+ neither the angels of the heavens nor the Son, but only the Father.+ 37 For just as the days of Noah were,+ so the presence* of the Son of man will be.+ 38 For as they were in those days before the Flood, eating and drinking, men marrying and women being given in marriage, until the day that Noah entered into the ark,+ 39 and they did not know until the Flood came and swept them all away,+ so the presence of the Son of man will be. 40 Then two men will be in the field; one will be taken along and the other abandoned. 41 Two women will be grinding at the hand mill; one will be taken along and the other abandoned.+ 42 Keep on the watch, therefore, because you do not know on what day your Lord is coming."
The point of comparison Jesus was making between his presence and the days of Noah is that people then did not know in advance when the flood would come, just as people would not know in advance the day of Jesus' presence.
His reference to the people in Noah's day doing everyday activities like marrying, eating, drinking, etc, is a way of emphasizing the unexpected nature of the event - they had no clue what was about to happen as they were caught doing the activities of normal, peaceful, everyday life. He mentions this to impress on his followers mind just how sudden and completely unexpected his presence will be, catching everyone by surprize. He does this by likewise referring to people being out working in the field and grinding at the mill - normal, everyday activities - on the day of his sudden presence. His whole point was to impress upon their minds that sudden and unexpected nature of his presence and thus reinforcing his main theme to keep on the watch because you do not know when he is coming.
Watchtower mistranslates the beginning of verse 39 by rendering it as "they took no note" as if to give the impression that they were not taking note of the sign of Noah's preaching as they were too caught up with everyday life. But's that's not the point Jesus was making. Jesus was simply emphasizing the unexpected nature of his presence to bolster his point about the need to keep on the watch.