Responsa for Yevamot 72:6
אין הולד של קיימא כו': תנא משום רבי אליעזר אמרו יוציא בגט
BUT IF THE CHILD IS NOT VIABLE etc. A Tanna taught: It has been said in the name of R. Eliezer that<span class="x" onmousemove="('comment',' Contrary to the law of our Mishnah which allows the levir to continue his connubial association with his sister-in-law wherever the child is not viable. ');"><sup>16</sup></span> he must put her out by means of a letter of divorce.<span class="x" onmousemove="('comment',' Though the death of the child has proved retrospectively that the levirate marriage was lawful, divorce is imposed upon such a union as a penalty for contracting it at a time when, owing to the uncertainty of the result of the pregnancy, it was of doubtful legality. ');"><sup>17</sup></span> Said Raba: R. Meir and R. Eliezer taught the same law.<span class="x" onmousemove="('comment',' Lit., 'said one word', that the penalty of divorce is imposed upon any union the legality of which was doubtful at the time the marriage was contracted. ');"><sup>18</sup></span>
Teshuvot Maharam
A. If L refuses to accept a divorce, Rabbi Eliezer is permitted to marry another woman. Rabbenu Gershom had intended that his prohibition against bigamy should apply only when the first marriage was legitimately contracted; but he had not intended to protect by his takkanah the marriage of a loose and immoral woman. Those who impute such intentions to Rabbenu Gershom will receive severe punishment from Heaven for defaming this saint's character and malignantly ascribing to him silly ordinances in order to make him appear ridiculous and thus discredit his other takkanot. Thus, the Rabbis are intent upon breaking up the marriage of a man to a woman pregnant by another. Would, then, Rabbenu Gershom protect such a marriage by his takkanah! I am even inclined to the opinion that the betrothal itself was invalid since the bridegroom was ignorant of facts, the knowledge of which would have prevented him from marrying L. There is no doubt, however, that Rabbi Eliezer is permitted to marry another woman in case L refuses to accept a divorce.
SOURCES: Cr. 161.