A key part of Wills' line of reasoning was that the Bible doesn't really prohibit abortion, not specifically. In fact, the Old Testament book of Numbers (verses 5.11-27) has God telling Moses:
If any man's wife go aside, and commit a trespass against him,
And a man lie with her carnally, and it be hid from the eyes of her husband, and be kept close, and she be defiled, and there be no witness against her, neither she be taken with the manner; And the spirit of jealousy come upon him, and he be jealous of his wife, and she be defiled: or if the spirit of jealousy come upon him, and he be jealous of his wife, and she be not defiled:
Then shall the man bring his wife unto the priest, and he shall bring her offering for her, the tenth part of an ephah of barley meal; he shall pour no oil upon it, nor put frankincense thereon; for it is an offering of jealousy, an offering of memorial, bringing iniquity to remembrance ... and the priest shall have in his hand the bitter water that causeth the curse: And the priest shall charge her by an oath, and say unto the woman, If no man have lain with thee, and if thou hast not gone aside to uncleanness with another instead of thy husband, be thou free from this bitter water that causeth the curse: But if thou hast gone aside to another instead of thy husband, and if thou be defiled, and some man have lain with thee beside thine husband ... Then the priest shall charge the woman with an oath of cursing, and the priest shall say unto the woman, The LORD make thee a curse and an oath among thy people, when the LORD doth make thy thigh to rot, and thy belly to swell; And this water that causeth the curse shall go into thy bowels, to make thy belly to swell, and thy thigh to rot: And the woman shall say, Amen, amen ... And when he hath made her to drink the water, then it shall come to pass, that, if she be defiled, and have done trespass against her husband, that the water that causeth the curse shall enter into her, and become bitter, and her belly shall swell, and her thigh shall rot: and the woman shall be a curse among her people.
Today, we would say the priest is coercing this hypothetically "defiled" woman to use a herbal abortifacient ("bitter water") to terminate her pregnancy! All because the woman's "defilement" has been cast as a "trespass" that has been committed against ... her husband!
In other words, the woman's sexuality, fertility, and reproductive capacity here belong to her husband, not to her. When she is "defiled" (actually, or according to her husband's suspicions) it is as if her (suspected) illicit lover has stolen property ... from the husband. This, the worst possible sort of transgression against the husband, demands that the woman (not the man she has supposedly "lain with") be punished in a most extreme way: "the curse shall enter into her, and become bitter, and her belly shall swell, and her thigh shall rot: and the woman shall be a curse among her people."
The society in question was a patriarchal one in which a female's reproductive capacities are originally owned, when she is still a child, by her father. We still say, do we not, that when a woman is married, her father "gives her away" to her new husband.
In such a view of womanhood, the menfolk of the tribe or society participate in a sort of communal "anti-defilement covenant." Together, the men control each and every woman's sexuality from the cradle to the grave.
According to such a covenant, a woman's sexual chastity is a precious possession, to be guarded at all costs by its possessors: the men. The men collaborate to set up the rules of conduct and official ceremony by which this can reliably be done. In the case of the ancient Hebrews, the rules were said to have come to Moses from God.
The underlying notion is, again, that a woman's sexual and reproductive identity belongs to her husband, her father, her brothers (whose job it is to avenge any adultery or rape committed by or against her), and the tribe's menfolk in general (who are adjured, for the sake of domestic tranquility, to rein in their natural tendency to want to "lie with" this woman or that, and with the other as well).
Looked at in one way, this kind of anti-defilement covenant puts every woman on a pedestal. That doesn't sound so bad. But looked at another way, it makes her into a mere object, a container of fertility by means of which her legitimate husband can reliably secure his own posterity.
This patriarchal idea of womanhood is so crucial to the mores of the Bible that womanhood's "defilement" becomes perhaps the only imaginable God-sanctioned rationale for abortion. On the other hand, an undefiled woman who is pregnant by her husband presumably must never have an abortion, for in so doing she is robbing her husband of his posterity.
An interesting aspect of this patriarchal setup is that it may also explain why the Old Testament comes down so hard against homosexuality.
On the one hand, it might be thought that any man who prefers other men to women would be, as one who is not at all inclined to "defile" the womenfolk, welcomed.
But, as a moment's consideration shows, such a man cannot really be part of the covenant, for the covenant is all about the sacrificial obligations men undertake vis-à-vis other men. A man bound by the covenant is giving up something big: his chance to sow his wild oats. He's making a huge sacrifice. (Perhaps this is why circumcision, the painful sacrificing of foreskin tissue at the tip of the penis, betokens membership in the covenant.)
So a shared sacrifice is what binds the covenanters indissolubly together. A homosexual man is making no real sacrifice by agreeing not to "lie" indiscriminately with the womenfolk. That's not very strong glue, by any means.
We who are Christians or Jews today inherit the ideas about womanhood that were inherent in the original, patriarchal covenant of anti-defilement, and transmitted to us via our Bible and our religious doctrines. But these ideas aren't a good match with our own up-to-date, feminist-inspired notions of womanhood. Feminism and women's liberation have taught us that a woman's sexual and reproductive capacities are her own, to do with as she sees fit.
In a society such as ours, where abortion-on-demand has recently (1973) been made legal, is it any wonder that religious conservatives are absolutely opposed to Roe v. Wade? The idea that a woman can, on her own, choose to have an abortion — or not — is diametrically opposed to the ancient understanding of womanhood implicit in the anti-defilement covenant. According to that understanding, a woman's fertility belongs to the menfolk in her tribe. Only they can adjudicate when an abortion is necessary.
And it is likewise no surprise that religious conservatives are equally adamant that homosexuality will not be tolerated — much less blessed by allowing gays to marry.
It all goes back to ancient ideas about how a tribe or society ought to treat the generative capacity of women: as something belonging to, and in turn demanding costly sacrifice from, men.