Is having sex with a woman while she is having her period (menstrual cycle) a sin in the Bible? I know it is mention somewhere in the Old Testament that it is a sin. What does the New Testament say?
Under the Old Testament, the Israelites were told the following:
"If a woman has a discharge, and the discharge from her body is blood, she shall be set apart seven days; and whoever touches her shall be unclean until evening. Everything that she lies on during her impurity shall be unclean; also everything that she sits on shall be unclean. Whoever touches her bed shall wash his clothes and bathe in water, and be unclean until evening. And whoever touches anything that she sat on shall wash his clothes and bathe in water, and be unclean until evening. If anything is on her bed or on anything on which she sits, when he touches it, he shall be unclean until evening. And if any man lies with her at all, so that her impurity is on him, he shall be unclean seven days; and every bed on which he lies shall be unclean" (Leviticus 15:19-24).
"According to the doings of the land of Egypt, where you dwelt, you shall not do; and according to the doings of the land of Canaan, where I am bringing you, you shall not do; nor shall you walk in their ordinances. ... Also you shall not approach a woman to uncover her nakedness as long as she is in her customary impurity ... Do not defile yourselves with any of these things; for by all these the nations are defiled, which I am casting out before you. For the land is defiled; therefore I visit the punishment of its iniquity upon it, and the land vomits out its inhabitants. You shall therefore keep My statutes and My judgments, and shall not commit any of these abominations, either any of your own nation or any stranger who dwells among you (for all these abominations the men of the land have done, who were before you, and thus the land is defiled), lest the land vomit you out also when you defile it, as it vomited out the nations that were before you" (Leviticus 18:3, 19, 24-28).
"If a man lies with a woman during her sickness and uncovers her nakedness, he has exposed her flow, and she has uncovered the flow of her blood. Both of them shall be cut off from their people" (Leviticus 20:18).
The New Testament makes no mention of rules concerning a woman's menstruation. However, before we can dismiss these rules as gone, Take a look at Leviticus 18 again. I quoted the larger context so you can see why God gave this rule along with several others. God charges the nations as defiling themselves because they had done things, such as having sex when a woman was having her blood flow. These nations were not under the law of Moses, yet God still calls these actions abominations. "For when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them)" (Romans 2:14-15).
The fact that God did not restrict the application of these laws to only the Israelites has caused many Christians to pause and wonder there is something more to be considered. It is because of the larger application made by God that I'm not absolutely certain that having sex with your wife when she is having her menstrual cycle is acceptable to God. Since we are warned, "Do you have faith? Have it to yourself before God. Happy is he who does not condemn himself in what he approves. But he who doubts is condemned if he eats, because he does not eat from faith; for whatever is not from faith is sin" (Romans 14:22-23), I personally have chosen to avoid the issue. A few days without sex won't harm a person, and it is beneficial not to act from uncertainty.