the gemara's interplay between the halacha before matan torah and after matan torah is curious. after all, tamar was not chayav seraifa if we apply the din torah, yet the gemara ascribes a q/a series between yehuda and tamar which would only apply after matan torah. do you have any insights?
dmartin, raanana eretz yisroel
Yehudah dealt with himself b'Dinei Yisrael and with Tamar b'Dinei Ben-Noach.
D. Z.
Your question is that she was Asur to him as his daughter-in-law (who is Asur to her father-in-law even after the death of his son, her husband). The Gemara says that she made sure that she was permitted to him by pointing out that she was a Yesomah. Since she was a Yesomah, her father did not accept Kidushin for her, and thus her Kidushin was not mid'Oraisa.
(As for our Gemara, in which Tamar has to excuse away her marriage to Er and Onan by saying she was a Yesomah, our Gemara might be following the opinion of Abba Shaul, that Yibum must be done Lishmah and not for any ulterior motive, Yevamos 39b. Since Yehudah did not know it was a Yibum, it was only permitted for him since she was not really his daughter in law.\
Many answers can be found in the Mforshei Hamikra to this question as well as why there is no Erva of Shomeres Yavam.
[Regarding what you write, "The Gemara says that... she was a Yesomah..." --] However Rashi on the Chumash (Breishis 38:24 D"H Vsisoreif) says that she was the daughter of Shem (in other words a Gdolah, the Pirush of the Sifsei Chachomim there needs Iyun from the Rashi on the Gemara) and yet in Possuk 26 he brings also the opinion that Lo Yosaf means he didn't stop.
Please note that the Ramban tells us that in actuality Yehuda did Yibum with Tamar since before Matan Torah the Mitzvah of Yibum was done by other relatives and not only a brother-in-law. Thus the Mitzvah of Yibum was being done by Yehauda although Tamar was his daughter-in-law. Therefore she was not forbidden to him.
Bpashtus because she didn't want him to know who she was, (Otherwise why didn't she tell him, that way he would be able to be Mkayeim the Mitzvah).
However why doesn't Rashi learn that after she told Yehuda she was a Pnua, he asked her perhaps her father was Mkadeish her to someone (and she may not even know whom, hence she is an Eishes Ish) to which she replied that I Am a Yesoma, meaning that her father died before she was born, hence he could never be Mkadeish her, and yet it would not be any contradiction to Shem being her father, (I realise that the Chashash of Shema Kidsha Oviha is a
Chashash Rochok, but Heicha Defsher Livrurei he tried, at least that way there is no Stira from this Gemara to the Midrash).
Yitzchok Zirkind