Is sex on the Sabbath a sin?
The Sabbath was the day of worship for the Israelites living under the Old Covenant. A part of the requirements for observing the Sabbath was the avoidance of work. "Observe the Sabbath day, to keep it holy, as the LORD your God commanded you. Six days you shall labor and do all your work, but the seventh day is the Sabbath of the LORD your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your ox, nor your donkey, nor any of your cattle, nor your stranger who is within your gates, that your male servant and your female servant may rest as well as you. And remember that you were a slave in the land of Egypt, and the LORD your God brought you out from there by a mighty hand and by an outstretched arm; therefore the LORD your God commanded you to keep the Sabbath day" (Deuteronomy 5:12-15).
The observance of the Sabbath day is not a part of the New Covenant. Christians worship on the first day of the week (Acts 20:7; I Corinthians 16:1-2) and not on the seventh day of the week. Nor are there regulations in the New Testament requiring Christians not to work. The rest from work, in part, gave Israel a chance to remember that they were once slaves who worked every day of the week. Not all Christians' ancestors were slaves rescued by God, so such a memorial would not make sense. Instead Christians keep a memorial meal, the Lord's Supper to remember our Saviors death upon the cross. "For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me." In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me." For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes" (I Corinthians 11:23-26).
Therefore, whether sex was allowed on the Sabbath for an Israelite is a moot point today since that law is no longer in effect.