Genesis describes sex with a variety of terminology:
1) The command to “be fruitful” (Genesis 1:28) implies sex.
2) Adam “knew” (yada’) Eve and she conceived (4:1, 25). The same term is used for Cain (4:17), for the Sodomites who attack the angels (19:5). “Not known a man” is a description of virginity (19:8; 38:16). Judah no longer “knew” Tamar after she became mother of his son/grandson (38:16).
3) Men “take a wife,” a phrase that implies sexual activity but doesn’t directly name it (4:19; 6:2; 24:67; 25:1; 28:1).
4) “Lie with” (shakav) is first used in connection with Lot’s daughters (7x in 19:32-35). Jacob lies with Leah when she gives her mandrakes to Rachel (30:15-16). Shechem lies with Dinah (34:2, 7), and Reuben with Bilhah, his father’s concubine (35:22).
5) Men “go in to” or “enter” (bo’) women, a verb that more likely refers to “entering her tent” rather than sexual penetration (16:2, 4; 29:23, 30; 30:3, 4; 38:8, 16, 18).
6) Occasionally, two of these terms are used together: “take” and “lie with” (34:2) and “take” and “go in” (38:2).
7) There are a few references to “harlotry” (zanah), in the stories of Shechem and Dinah (Genesis 34:31) and Judah and Tamar (38:15, 24).
Several things emerge from this summary.
1) Apart from zanah, the language doesn’t give any indication whether the sexual activity is good or bad. “Know” can describe either the marital, procreative sex of Adam and Eve, or the homosexual desires of the men of Sodom.
“Lie with” tends to have a negative connotation. It refers to incest (Lot’s daughters; Judah and Tamar) and perhaps something close to rape (Shechem and Dinah, though the relation may be consensual). Potipher’s wife attempts to seduce Joseph by demanding “Lie with me” (39:7), and she accuses him of rape with the same term: “he came in to lie with me, and I screamed” (39:14). Even when it describes marital sex, it has something smarmy about it, as when Leah buys a night with her husband (Genesis 30:14-24).
No wonder we have difficulty sorting out the judgment of Genesis on the sexual activity of the patriarchs. There is little distinctive terminology for illegitimate sex, no uses in Genesis of na’aph, “commit adultery” (first use in Exodus 20:14).
2) The terminology typically describes male sexual activity. Adam knows Eve; we’re not told that she knew him. Men “enter” women, and men “lie with” women, and not vice versa. Usually, but not always. Lot’s daughters take the initiative to “lie with” their father (19:33). When Leah purchases a night with Jacob, Jacob’s the one who “lies with” and “enters to” Leah, but Leah has taken charge: “You must come in to me,” she tells him (30:16).
It would be intriguing to trace this asymmetry through the rest of the Bible, and see if it’s consistent. On the face of it, there seem to be notable exceptions that aren’t incestuous. Ruth “enters” (bo’) to Boaz, and “lies” (shakav) at his feet (Ruth 3:7-8), mimicking Lot’s daughters while also undoing their incest. And the Bride of the Song of Songs takes sexual initiative at various points in the poem.
3) At crucial points, conception occurs in the absence of any of this terminology of sexual activity. Abram “goes in to” Hagar to father Ishmael, but Sarah conceives without Abraham knowing, lying with, taking, or going in to her (Genesis 21:1-2). Sarah’s child comes from Yahweh, and He is also the one to open the wombs of Rebekah (25:21), Leah (29:31), and Rachel (30:22-23). In each case, there’s no reference to male action. These conceptions come directly from God.
In reality, we assume, Abraham, Isaac, and Jacob knew, enter to, and lay with their wives. But given the text’s silence, one is tempted to suggest that these are miracle sons, given without male mediation.