The source of having a Mechitza to separate between men and women is found in a Baraisa in Meseches Sukkah, which describes the Simchas Beis Hashoeiva celebration in the Beis Hamikdash.
“Originally they would have the women standing on the inside (the Ezras Nashim) and the men on the outside (on the Temple Mount), but this led to Kalus Rosh (inappropriate frivolous behaviour)... So, they enacted that the women should sit above and the men down below”. The Tosefta elaborates, describing how they would make elevated galleries for the women to stand on.
The Gemara questions how these galleries could be added, since every aspect of the Temple’s design had to be according to the instruction of Hashem through the Neviim? The Gemara answers that the sages found a Posuk which teaches that a separation between men and women is required. Zechariah prophesis- es about the future Hesped for Moshiach ben Yosef, that the “The land will eulogise in families. The house of David on their own and their wives on their own...”
The Maharsha adds that it would also be permissible to add these structures simply in order to prevent people coming to an issur.
A Mechitza for Davening
This Gemara is the basis for having a Mechitza in Shule. The need to have a Mechitza in a holy place at the time of davening is universally accepted as Halacha. Igros Moshe writes that this requirement in Mideoraisa, hence it could be added to the design of the Beis Hamikdash. Other Poskim view the Mechitza as a Rabbinic requirement.
At Other Gatherings
What about having a Mechitza at other gatherings such as a shiur or a Simcha, functions and concerts?
Concerning shiurim, the Mordechai refers to “the Mechitza at the time of the Drasha”. Halachic authorities including; the Maharil, the Ta”z, the Alter Rebbe, Mishna Berurah and Aruch Hashulchan, all refer to this Mechitza in the Laws of Shabbos. This type of temporary wall may be erected on Shabbos because it is put up purely for reasons of Tznius.
The source of this idea of having a Mechitza at a Drasha is based on the Gemara which describes how Abaye would set up ceramic barrels between the men and the women. Rashi says that this was at the time that they came to hear the Drasha.
Concerning a Mechitza at other gatherings, in one Teshuvah, Igros Moshe writes that it may be required. A proof that a Mechitza is needed at such gatherings is brought from the very Pasuk used by the Gemara to derive the requirement of a Mechitza, which is referring to a Hesped rather than to prayer.
However, in a subsequent Teshuvah, Igros Moshe writes that it appears that a Mechitza is not necessarily required. He brings a proof from the fact that men and women would eat the Korban Pesach together and a Mechitza was not required. On the contrary, a Mechitza would be problematic because the Korban Pesach must be eaten as one group.
Rav Moshe’s position is clear in later Teshuvos. All events which are open to the public require a Mechitza. A wedding however is a private (invitation only) affair and therefore a Mechitza is not required. Since a shiur is open to the public, a Mechitza is required min hadin. For Kiruv activities where there is no other option, and people will not come if there was a Mechitza, he is more lenient.
The Seridei Eish rules that due to present day circumstances, at gatherings such as weddings, speeches and Drashos, a Mechitza is not required. It is sufficient that the men and women sit separately. The Mordechai is describing an extra practise of Midas Chassidus (piety).
Other Poskim who require a Mechitza at all gatherings where men and women are attending include; the Shevet Halevi, Divrei Yoel and Be’er Moshe.
Interestingly, the Be’er Moshe writes that in Europe a Mechitza as not always used, but this was because the women kept to a very high level of Tznius in dress. Nowadays however a Mechitza is required.
Divrei Shalom distinguishes between Simchas and similar gatherings where there is a concern of Kalus Rosh and a Mechitzah is required min hadin, and a shiur where a Mechitza would only be a Midas Chassidus since it’s an atmosphere of Yiras Shamayim.
The Lubavitcher Rebbe
There is a famous letter of the Rebbe on this topic. The letter referred to a shiur being given in Russia before the fall of Communism. Due to the unique needs of that time and place, the Rebbe gave an allowance and permitted the shiur to take place without a Mechitza. Even still, the Rebbe stresses that the men and women should still sit separately.
Dvar Melech records the response of the Rebbe to a Shaliach who wished to host a mixed dinner, so as not to push away potential donors. The Rebbe replied, “Why should you push away certain Yiras shamayim (affecting all of the participants) on account of a Safek in extra money (from only a few people)”. See handwritten response below.
The singer Mordechai Ben David relates that when the first HASC concert was being organised, ticket sales were low, and they asked the Rebbe if they could have a mixed “family” section to encourage more attendees for this charity event. The Rebbe responded, “all that is written above does not change the Shulchan Aruch”. In a similar case, an oganisation asked the Rebbe if they could have a mixed concert, writing that they would incur a financial loss if the seating was completely separate. The Rebbe responded, “Mixed seating is absolutely prohibited; The loss of money is only a doubtful possibility. The absolute has precedence...”
Conclusion
Whilst there are halachic authorities who permit separate seating at shiurim without requiring a Mechitza, there is certainly a Midas Chassidus and according to many authorities a halachic obligation to have a Mechitza. At Simchas there may even be stronger case for requiring a Mechitza due to the stronger possibility of Kalus Rosh.
Bnei Torah and Charedi Kehillos should certainly be strict in this regard.
For Chabad Chassidim and institutions under the auspices of Chabad, especially in light of the view of the Rebbe’s letter and responses quoted above, it is clear that they are required to have completely separate seating, and ideally a Mechitza, at all gatherings where both men and women are present, including shiurim, Simchas, Kiddushim, events and concerts.
