Whether the Hebrews would have referred to Perez as Tamar’s seed or Judah’s seed was not the point I was trying to make. My point was that the promise was not just dependent upon the man, but every bit as much through the woman. And sometimes God used seemingly unconventional circumstances to bring the right man and woman together before Christ’s particular family line could be achieved.
You may be right about the gender of zarah (“zerah” in that particular verse, I think). It has been about seven years since I last translated that chapter of Genesis, so I’ll have to go look up my notes again, I suppose. I seemed to remember that zarah–while having the appearance of a feminine noun–was actually irregular. But I could be wrong; like I said, I’m a little rusty in my ancient Hebrew. If you’ve got a good lexicon, then I’ll defer to it. My lexicon of choice is the Brown-Driver-Briggs (although it does contain a few minor misprints here and there, at least in the edition I was using seven years ago).
It may be that [Tamar] was under obligation to bring forth a son in honor of the deceased Er – and therefore by marrying another, or whoring, she became liable. Again, the two are one flesh may be seen as terminated upon sexual intercourse with another after death. Judah, on the other hand may not have considered it at all sinful that he had fornicated – but the cloak and staff – the items of a shepherd – recall that Judah is the one who was responsible for throwing Joseph in the pit, planning to kill him, and tricking his father by the blood of a kid with Joesph’s cloak. He also is the one who refused to fulfill the promise of raising up a child to his firstborn – Er – by denying her the third son in marriage by a lie.
The Levirate marriage custom to which you are referring was a responsibility that fell upon the men in the family, and the right of the widow. It was the responsibility of the next single brother in line to take his brother’s widow as his wife, in order to conceive a son that would be legally regarded as the deceased’s own. If the brother were to refuse, the widow would publicly denounce him before all the elders, and the brother’s family would be forever shamed. It was not the widow’s fault if the brother or next-of-kin should refuse to take her (although widows had precious few legal rights without a man to care for them, so I would imagine that any woman would be hard-pressed to give up on finding a go’el (“redeemer”) who would help her to conceive a child).
In the case of Tamar, she was Er’s widow, and then was taken by Onan, as the marriage custom demanded; but by “spilling his seed on the ground,” Onan refused to allow Tamar to conceive (and implicitly refused to honor the Levirate custom), so God killed him. By law, Shelah was now required to marry Tamar. But Judah insisted that he was too young, so he promised Tamar that she could marry Shelah when he was more mature, so that Er could have a son to his name. That, of course, did not happen. But Tamar was determined to force her father-in-law to honor his promise that she would have a son for Er. Thus, she seduced him by posing as a temple prostitute (a distinctly pagan practice, but Judah didn’t seem to mind). By taking Judah’s staff and seal, Tamar was gathering proof of his identity, as both items would have been clearly marked with the mark of the House from whence the owner came. (The parallels you make between these items and the episode with Joseph are fascinating, although I wonder if that might be reading a little much into the text. The seal and cord, for instance, were not the same thing as a cloak, and staves were not specific to shepherds alone.) She used these as leverage against Judah when he tried to have her killed for “playing the harlot,” for she knew that these items would clearly identify him as the father. This is why Judah understands that Tamar was in the right, because she was simply taking from Judah what he had promised her to begin with–her rights as a widow through the Levirate custom, and a child for her deceased husband.
The conditional necessity I see for Ruth is one of genetics. Jesus, as we know him, came into the world as a descendant of David, the son of Jesse, the son of Obed, the son of Boaz and Ruth. In fact, the whole story of Ruth is all about the fulfillment of the Levirate marriage custom. Naomi’s sons had all died, so her husband had no heir. She was too old to have children, so Ruth–her daughter-in-law–had to take on the responsibility of finding a go’el who would redeem their family line by providing an heir. Boaz graciously accepts that role. What is important to remember is that Boaz’s firstborn would not have been his own, but would have been legally regarded as Elimelech’s heir (cf. Ruth 4:14). Ruth was the only one who could conceive a child for his line–thus, she was necessary. What is truly interesting to me, though, is that the Toledoth of Christ’s genealogy in the Gospel of Matthew refers to Obed as the son of Boaz, which would not have been technically correct. Genealogies were supposed to be traced through the legal ancestor, not the biological one. At any rate, Scripture contains a great many paradoxes. The fatherhood of Boaz is just one of them, so I try not to think too much about it.
It is true that the prophecy was made to a woman; Eve; Which highlights the feminine role in bearing children; but do you see more than that or just a correlation?
All I can say is that I believe that the Woman plays a crucial role in salvation history, and I do believe that God meant it to be so. Christ refers to Mary as Woman in the Gospels to show how she is connected to Him, just as Eve was connected to Adam. The Church Fathers said as much, and I am inclined to agree with them.