“and be unclean.” According to Levitical law, when a man and woman had normal sexual intercourse it made them “unclean.” This is not saying that sexual intercourse is somehow frowned upon by God. Saying that would be like saying that if a loved one died, God frowned upon any of His people touching the dead body or giving it a decent burial because that made the people involved unclean. God wants us to understand that there is a difference between His holiness and our common earthly life, and there are things in this life that made people “unclean” in the sight of God but were simply part of this fallen world. People were to have such respect for God that they recognized that He is somehow set apart from their daily earthly life, and although He is intimately involved with people on a minute-by-minute basis, when they “come into His courts” to worship, there is a holiness there above one’s simple daily routine.
However, when the Christian Church started on the Day of Pentecost (Acts 2), the Church became the Temple of God (1 Cor. 3:16), and the physical Temple and sacrifices stopped being performed by Christians (although that took some time). Today, because of the work of Christ, Christians have a different relationship with God than the Jews of the Old Covenant had, and there is no rule saying a Christian should not enter a church building if he or she has engaged in sexual intercourse.
An Israelite man or woman who was unclean because of normal sexual intercourse could perform almost all of the normal tasks of the day. The major thing being unclean prevented was going into the court of the Tent of Meeting or the Temple. We can see the pattern of sexual intercourse making someone unclean in verses such as Exodus 19:15 when Israel was about to meet God, and Leviticus 16:29-31, which is about people’s ritual purity on the Day of Atonement (cp. Lev. 12:4 about uncleanness due to childbirth).
One very important result of God’s separating His holy precincts of the Tent of Meeting and the Temple from uncleanness due to sexual intercourse was it effectively forbade the practice of temple prostitution. Cultic prostitution was a pagan practice that existed in many parts of the ancient world at various times in which female and male “prostitutes” were part of, or somehow associated with, the worship of a pagan god (some modern scholars rename “temple prostitution” as “sacred sex” due to the fact that the “prostitute” may not have been paid. The name is not important, but we would point out that there was some sort of payment, even if it was to procure the favor of a god or king). The Mosaic Law specifically forbid temple prostitution or sacred sex (Deut. 23:1-18), but saying that someone who had sexual intercourse was “unclean” was another way to ensure that it never became part of the worship associated with God’s Temple.