Bereshit 4:1 בראשית ד׳:א׳
1 א

והאדם ידע את חוה אשתו. אחר שראה שנטרד מגן עדן בחטאו ונקנסה עליו מיתה ולא יחיה לעולם הוצרך להזדווג עם חוה לקיום המין להשאיר אדם אחריו. ודע כי התשמיש בלשון התורה נקרא ידיעה והטעם לפי שהזרע בא מן המוח שהוא מקום החכמה והדעת. ועוד תאות התשמיש היתה סבתה עץ הדעת ולכן נקרא בשם ידיעה.

והאדם ידע את חוה אשתו, “and man had been intimate with his wife Chavah, etc.” After Adam had realized that he had been expelled from Gan Eden, that death had been decreed for him, and that he would not live indefinitely, it became necessary for him to have sexual relations with his wife in order to assure himself of children for posterity. You should remember that the Torah usually calls the act of marital union ידיעה, “(carnal) knowledge.” The reason for this is that the seminal fluid of man originates in the brain, the seat of his wisdom and knowledge. In addition to this, the desire to engage in sexual intercourse was the result of man having eaten from the tree of “knowledge.” Hence the use of the word ידיעה, “knowledge,” to describe such relations is most appropriate.