Just a guess on my part. God designed sex to be a loving act who’s purpose was to procreate as well as being pleasurable to the husband and wife. Since he commanded to be fruitful and multiply…the baby’s birth would be painful for the woman. Remember that he also punished the man.
17Then to Adam He said, “Because you have listened to the voice of your wife, and have eaten from the tree about which I commanded you, saying, ‘You shall not eat from it’;
Cursed is the ground because of you;
In toil you will eat of it
All the days of your life.
18“Both thorns and thistles it shall grow for you;
And you will eat the plants of the field;
19By the sweat of your face
You will eat bread,
Till you return to the ground,
Because from it you were taken;
For you are dust,
And to dust you shall return.”
And thy conceptions. Septuagint, “thy groaning.” The multifarious sorrows of childbearing, must remind all mothers (the blessed Virgin Mary alone excepted) of what they have incurred by original sin. If that had not taken place, they would have conceived without concupiscence, and brought forth without sorrow. (St. Augustine, City of God xiv. 26.)
— Conceptions are multiplied on account of the many untimely deaths, in our fallen state. Power, which will sometimes be exercised with rigor. (Haydock)
— Moses here shews the original and natural subjection of wives to their husbands, in opposition to the Egyptians, who, to honour Isis, gave women the superiority by the marriage contract. (Diodorus i. 2.) (Calmet)
3:16 Toil…pain: the punishment affects the woman directly by increasing the toil and pain of having children. He shall rule over you: the punishment also affects the woman’s relationship with her husband. A tension is set up in which her urge (either sexual urge or, more generally, dependence for sustenance) is for her husband but he rules over her. But see Sg 7:11.