‘And Moses went down from the Mount to the people and sanctified the people, and they washed their clothes. And he said to the people, “Be ready against the third day. Do not come near a woman.” '
The sanctifying by Moses was possibly by the offering of sacrifices (compare Exodus 29:1) after which the people washed their clothes in readiness for God's call. From then on they had to avoid anything that would make them ritually unclean, including sexual intercourse. Sexual intercourse produced uncleanness on the body (Leviticus 15:16-18) and they were required to be ritually pure for the whole three days (compare 1 Samuel 21:5).
It is significant that sexual intercourse was seen as an earthly activity excluded from the heavenly. This may well partly have been because of its association with certain religious rituals elsewhere, as well as because it was associated with man's first sin. But Jesus will later point out that the angels in Heaven ‘neither marry nor are given in marriage'. It is very much a ‘fleshly' activity.