And he said to the people, “Be ready for the third day; do not go near a woman.”
Exodus 19:15 ESV

There is nothing sinful about being with your wife, but sexual activity makes one tamai (ritually unclean) and unsuitable for close approach to God. The preceding warnings indicate that this prohibition was for their physical safety, not for an object lesson or abstract moral principle with no tangible effect on the real world.