Kesubos 3b - Tell the Tznuos that O'nes is Mutar

2 points:

1. In the ha'va aminah of the gemara that sakana means a gezeirah to kill anyone who gets married on wednesday, and therefore asks that chazal should uproot the entire takanas chachamim of limiting marriage to a particular day (see rashi and tosafos who seem to disagree if they should institute another day, or leave it open to a few options). However, when the gemara switches to the sakana being a gezeira to have relations with the hegmon, the gemara assumes that there is not reason at all to uproot the takana since the relations will be b'oneis (the gemara only follows up with the question of uprooting the takana since it will lead to sakana for tzenuos). This seems to compiment the rashi on 3a that understood from the pasuk of "u'lna'arah lo ta'aseh davar", that o'nes is not simply a p'tur from punishment, but it is as if the act was never done; here too it is as if the act of z'nus was never committed so there is no need to uproot the takanah.

2. Major machlokes between rabbeinu Tam and the R"i what exactly is the heter to commit adultery b'ones, since it is a yei'hareg v'al ya'avor. R"i understands that the heter is karkah olam, meaning, that since she is passive, she is not doing an issur. It is not so clear if the o'nes is required to compliment the aspect of karka olam to be matir. Although in this particular case there is o'nes and karkah olam, by both yael and esther (ka'asher avaditi) they did it willingly, yet Tosafos claims that they had a heter of karka olam. Perhaps their heter was a combination of karka olam and the hatzala of klal yisroel - needs iyun! One way or the other, it is certainly not a heter to be matir the woman to her husband, as we clearly find by esther that she became assur to mordechai when she "decided" to visit achashveirosh, even though it was mutar for her to do so.