The following is an excerpt from my article "The Mehitzah Controversy: 50 Years Later" in BD"D vol. 17, an article that generally defends the requirement of a mehitzah and the positions of R. Joseph B. Soloveitchik and R. Moshe Feinstein in particular. Note that this excerpt is from an unedited draft of the article:
While this essay has focused largely on reasons to require a mehitzah, it will now address a powerful, if ambiguous, case to permit or even require mixed seating in the synagogue. The laws of the Torah are eternally binding. However, while they are free from annulment by human hand there is an internal mechanism within the law by which it can be temporarily set aside in accommodation of a greater need. The classic example of such an occurrence was the abrogation of the prohibition against sacrificing on a private altar by Elijah on Mount Carmel. He saw that in order to save the Jewish people from mass defections to idolatry he had to forcefully and convincingly demonstrate the truth of his message, something he accomplished by temporarily setting aside a biblical prohibition. As the rabbis termed it: “’It is time for You to act, O Lord, for they have regarded Your law as void’ (Psalms 119:126) – Rabbi Nathan said: [This can be read as:] Void your law because it is a time to act for the Lord.” This overriding of one law so as to save the entire corpus has occurred more than once in history. Another prominent case was the writing of the oral law in the form of the Talmud, an act that is technically prohibited because the oral law must remain informal and unwritten. However, since the oral law was in danger of being forgotten, the leading rabbis of the time arrived at the bold conclusion that the law must be violated so as to be preserved. Maimonides codified this process in the following way:To see the whole picture, you really need to read the entire article.If [a court] sees fit to temporarily neutralize a commandment or violate a prohibition in order to bring the masses back to religion or to prevent many Jews from stumbling in other areas, it does whatever the times require. Just like a doctor will cut off someone's arm or leg so that his totality will live, so too the court can rule at certain times to temporarily violate some commandments in order that they will all [eventually] be fulfilled.Just like amputating a part of the body can save the whole so, too, there are rare occasions when setting aside one law can salvage the entire Torah. Thus, when rabbis were presented with the ultimatum that if they did not permit mixed seating the majority of synagogue members would leave for a Reform temple, they were faced with a colossal dilemma. As members of an antinomian temple, as Classical Reform synagogues can be justly termed, these congregants would certainly lose quickly whatever observances they once maintained and, perhaps more importantly, they would no longer be under the influence of a tradition-minded rabbi who might influence these Jews to return closer to their heritage. If, however, the rabbi accommodated the congregants’ desire for mixed seating he could guide these members’ progress or at least prevent a major decline in observance. With the dilemma presented in this fashion, the conclusion naturally emerges that it would be better to set aside the requirement for separate seating in order to stave a mass defection away from traditional Jewish observance.
For this reason, more than any other, the leading halakhists of the early twentieth century Conservative Movement begrudgingly permitted mixed seating on a case by case basis. They were cautiously willing to forgo the requirement of a mehitzah in the synagogue in order to keep thousands of Jews within the framework of a movement that generally respects halakhah.
There is yet room to question this conclusion and to argue that, given the context of the demand for separate seating, accommodationism was counter-productive; it encouraged an already strong desire to restructure Jewish law according to one’s own wishes rather than urging congregants to change their lives to fit halakhic demands. This was the Orthodox counter-argument. However, the ultimate conclusion of whether mixed seating was a necessary transgression or a destructive change to halakhah lay in the judgment of the halakhic decisor, a heavy responsibility for anyone to bear...
 1 Kings 18.
 Berakhot 54a. Cf. Berakhot 63a; Temurah 14b.
 Gittin 60a.
 Mishneh Torah, Hilkhot Mamrim 2:4.
 Prof. Boaz Cohen, Proceedings (supra note 11), pp. 140-141; Prof. Louis Ginzberg, Responsa (supra note 10), nos. 7-9. A careful reading of Prof. Ginzberg’s writings on the subject shows that he considered separate seating to be a binding custom that he hesitantly permitted only because of the spiritual danger in the situation of that time.
 For a passionate halakhic argument against the entire “time to act” justification, see R. Aaron Felder, She’elat Aharon (Philadelphia: 2000), nos. 3-4. I thank my dear friend R. Daniel Z. Feldman for bringing this to my attention.
 Even those who concede that, were the conditions appropriate, mixed seating could be temporarily permitted were still able to honestly testify that mixed seating is prohibited. If not, one would never be able to say that anything, even eating pig, is prohibited by Judaism because there exist extenuating circumstances, such as a life-threatening illness that requires the eating of pig meat, in which the forbidden is allowed.
 Cf. R. Moshe Ibn Habib, Kapot Temarim, Sukkah 34b on Tosafot sv. ve-lidrosh regarding on whom such decisions fall.