1 YHVH said to Moses and Aaron,
2 "Say to the people of Israel, When any man has a discharge from his body, his discharge is unclean.
3 And this is the law of his uncleanness for a discharge: whether his body runs with his discharge, or his body is stopped from discharge, it is uncleanness in him.
4 Every bed on which he who has the discharge lies shall be unclean; and everything on which he sits shall be unclean.
5 And any one who touches his bed shall wash his clothes, and bathe himself in water, and be unclean until the evening.
6 And whoever sits on anything on which he who has the discharge has sat shall wash his clothes, and bathe himself in water, and be unclean until the evening.
7 And whoever touches the body of him who has the discharge shall wash his clothes, and bathe himself in water, and be unclean until the evening.
8 And if he who has the discharge spits on one who is clean, then he shall wash his clothes, and bathe himself in water, and be unclean until the evening.
9 And any saddle on which he who has the discharge rides shall be unclean.
10 And whoever touches anything that was under him shall be unclean until the evening; and he who carries such a thing shall wash his clothes, and bathe himself in water, and be unclean until the evening.
11 Any one whom he that has the discharge touches without having rinsed his hands in water shall wash his clothes, and bathe himself in water, and be unclean until the evening.
12 And the earthen vessel which he who has the discharge touches shall be broken; and every vessel of wood shall be rinsed in water.
13 "And when he who has a discharge is cleansed of his discharge, then he shall count for himself seven days for his cleansing, and wash his clothes; and he shall bathe his body in running water, and shall be clean.
14 And on the eighth day he shall take two turtledoves or two young pigeons, and come before YHVH to the door of the tent of meeting, and give them to the priest;
15 and the priest shall offer them, one for a sin offering and the other for a burnt offering; and the priest shall make atonement for him before YHVH for his discharge.
16 "And if a man has an emission of semen, he shall bathe his whole body in water, and be unclean until the evening.
17 And every garment and every skin on which the semen comes shall be washed with water, and be unclean until the evening.
18 If a man lies with a woman and has an emission of semen, both of them shall bathe themselves in water, and be unclean until the evening.
Moses (or whomever the true author of this book was) believed sex to be unclean. This is ignorance. However, it was very popular among the Hebrew people and remains so among many Christian (and other bible believing) sects.
19 "When a woman has a discharge of blood which is her regular discharge from her body, she shall be in her impurity for seven days, and whoever touches her shall be unclean until the evening.
20 And everything upon which she lies during her impurity shall be unclean; everything also upon which she sits shall be unclean.
21 And whoever touches her bed shall wash his clothes, and bathe himself in water, and be unclean until the evening.
22 And whoever touches anything upon which she sits shall wash his clothes, and bathe himself in water, and be unclean until the evening;
23 whether it is the bed or anything upon which she sits, when he touches it he shall be unclean until the evening.
24 And if any man lies with her, and her impurity is on him, he shall be unclean seven days; and every bed on which he lies shall be unclean.
25 "If a woman has a discharge of blood for many days, not at the time of her impurity, or if she has a discharge beyond the time of her impurity, all the days of the discharge she shall continue in uncleanness; as in the days of her impurity, she shall be unclean.
26 Every bed on which she lies, all the days of her discharge, shall be to her as the bed of her impurity; and everything on which she sits shall be unclean, as in the uncleanness of her impurity.
27 And whoever touches these things shall be unclean, and shall wash his clothes, and bathe himself in water, and be unclean until the evening.
28 But if she is cleansed of her discharge, she shall count for herself seven days, and after that she shall be clean.
29 And on the eighth day she shall take two turtledoves or two young pigeons, and bring them to the priest, to the door of the tent of meeting.
Moses (or whomever the true author of this book was) believed a woman's menstrual cycle was sinful, as is demonstrated by these passages. This is pure ignorance. It is natural. As such, no atonement is necessary, and certainly does not require the involvement of a priest, a sin offering, or any other such thing. However, these beliefs were prevalent among the Hebrew people, as earlier passages demonstrate. Its ignorant, but it was (and even in some present day sects, still remains) popular.
30 And the priest shall offer one for a sin offering and the other for a burnt offering; and the priest shall make atonement for her before YHVH for her unclean discharge.
31 "Thus you shall keep the people of Israel separate from their uncleanness, lest they die in their uncleanness by defiling my tabernacle that is in their midst."
32 This is the law for him who has a discharge and for him who has an emission of semen, becoming unclean thereby;
33 also for her who is sick with her impurity; that is, for any one, male or female, who has a discharge, and for the man who lies with a woman who is unclean.
The indication here, beyond the practical, literal need for cleanliness, is that all of our life essence is sacred. Any of it, be it blood or the seed of copulation, relates to our life essence and is not to be wasted or misused. Doing so is treated as a sin by the Hebrew people. Again, this is pure ignorance. The fact is that while life essence IS sacred, its release in these instances is a natural part of living. It is NOT "unclean", "a sin" or any other such garbage.