We still need to explain two halakhot, one Torah law and one rabbinic, that seem to indicate that despite the sanctity of the mitzva of ona, it has an aspect of impurity as well.
According to the Torah, semen ejaculated by a Jew is a source of impurity (av ha-tuma). Even when a husband and wife have marital sexual relations and fulfill the Torah commandment, since he ejaculated inside her, they both become impure, as they have come into direct contact with a source of impurity (they have the status of rishon le-tuma). To purify themselves, they must immerse in a mikveh and wait until nightfall, whereupon they resume their ritually pure status and are permitted to enter the Temple, eat the meat of the sacrifices, and, if they are kohanim, eat teruma. Similarly, clothing and kelim that have come into contact with a Jewish man’s semen become ritually impure and may not be used or worn while handling sacrifices or objects that must retain their pure status. These items are purified through immersion in a mikveh. In the Torah’s words:
When a man has an emission of semen, he shall bathe his whole body in water and remain impure until evening. All cloth or leather on which semen falls shall be washed in water and remain impure until evening. And a woman with whom a man has sexual relations and an emission of semen, they shall bathe in water and remain impure until evening. (Vayikra 15:16-18)
Ritual impurity, tuma, is an expression of life lost. The supreme source of tuma (avi avot ha-tuma) is a corpse. The impurity of nidda also expresses death; this was a potential pregnancy that has been lost and is no longer. The impurity of semen is likewise an expression of something that could have created new life but has now been lost and has died (Kuzari 2:60-62). The Torah teaches us that even when semen is ejaculated in fulfillment of the mitzva of ona, it harbors impurity. Likewise, we find that childbirth renders a woman impure. The idea is that when any grand notion descends to this world, there is a certain aspect of death. The vision is always grander than its realization. In this case, the hopes preceding a birth are grandiose. There is a tendency to believe that the whole world will change for the better because of this birth, and that the newborn child will be absolutely perfect. In reality, after birth we return to routine life, with its aches, pains, and exhaustion. Despite the miracle of birth, the new baby will still need to contend with all the challenges of human life. Even our bodies sense this letdown, which is an aspect of post-partum depression.
A man, too, experiences letdown after ejaculation, even though it occurs within the holy context of the mitzva of ona. Beforehand, he believed that he will soon be closer than ever to his beloved wife, and that everything will always be great. His heart fills with passion and excitement, building until its release during ejaculation. But then he falls back to the routine of this world, his passion drained. In contrast, women do not experience this fall. After orgasm, her return to this world is gentle and easy. When their sexual union is loving and joyful, her feelings of satisfaction and contentment linger for a while. Therefore, the man’s semen is impure, but not the fluids secreted by the woman during arousal.
Perhaps the letdown that a man experiences after orgasm expresses human incompleteness. Even when one is truly in love, true unity between two people is fleeting. Even when a man truly wishes bring joy to his wife, he remains, to some degree, within himself. He fails to harness all of his passion to that love. If this were possible, there would be no more death in the world, no more post-coital tristesse, and no more post-partum depression. The Sages allude to this in their explanation of the impurity of childbirth:
“Indeed, I was born with iniquity; with sin my mother conceived me” (Tehilim 51:7). R. Aḥa explained: Even if a person is the most devout of the devout, it is impossible for him to be completely devoid of sin. David said to God, “Master of the universe! Did my father Yishai intend to create me? He had nothing in mind but his own pleasure! We know that this is true, for after they attend to their needs (i.e., after sexual intercourse), one turns this way and one turns that way (and they fall asleep). You, however, gather in every single drop (of semen, to ensure that there are viable fetuses).” This is what David meant when he said, “Though my father and mother abandon me, the Lord will take me in” (Tehilim 27:10). (Vayikra Rabba 14:5).