Gil Student has just raised the question of mixed seating at weddings. This has become something of a cause celebre in Israel (and I assume elsewhere). Here, at least, it has gotten to the point that receptions are held in separate (though adjoining halls), leading no a few people I know to abjure the wedding festivities and to make due with attending the Huppah (which in Israel is often a matter of 'mixed standing.') This reaction is highly problematic, since the main point of attending a wedding is to rejoice with the bride and groom. Indeed, according to Tosfos (quoting the Yerushalmi) a person who is attends the weding without dancing etc. is subject to celestial excommunication (Tosafot Pesahim 114a s.v. ואין).
In any event, I must demur from Gil's conclusion that 'separate seating has far more halachic support.' The mixed seating option is equally legitimate. The prohibition noted by Sefer Hasidim (hardly a purely halakhic source) may even be enlisted to indicate that in Tosafist Germany the weddings had mixed seating. In addition, I strongly agree with Rav YH Henkin's observation:
And with this I retract what I wrote... that at weddings it is proper to seat single men and single women separately even if the married couples sit together. This is [still] so with young men and women who are not yet ready to get married. However, regarding those who have reached that stage, to the opposite, it is a mitzvah so that they get to know each other in a place where there is no concern for yihud and each couple is not alone on a "date," as is done today...
When I got married, the mixed seating question came up. My wife's family has a lot of Haredim and mine is overwhelmingly secular. One side wanted separate seating with a mehitzah and the other wanted social dancing. Obviously, the latter was out of the question but so was the former (as far as I was concerned). Almost everyone has non-religious relatives and friends and the potential for Qiddush and Hillul HaShem is very high.
Anyway, I put the question to the Rov זצ"ל. His immediate reaction was, 'Don't make a mahloqes.' When I explained that I already had a mahloqes, he told me that there was nothing wrong with having mixed seating at the tables. (We had separate seating at the ceremony). We could include 'שהשמחה במעונו' in the Birkat HaMazon (contra Sefer Hasidim). The Rov (who at the time was supposed to be the Mesadder Qiddushin, but fell ill the day before the wedding) made it very clear that the entire setup, as I had described it, was perfectly fine. [On another occasion, he explained to me that his insistence on the separate seating a mehitza was an issue in the Sanctity of the Synagogue. Indeed, as I recall, there was no separate seating at the Saturday Night shiurim at Maimonides. Sunday mornings, there were women who came but they sat at a side table. Since I never attended a Maimonides School Dinner, I have no idea what happened there.]
Now, the reason why posqim often write that that their decisions are 'להלכה ולא למעשה' is because precedent is not binding in Halakhah. Each case is different and precedent should not be mechanically applied. Therefore, I am not relating this story in order to encourage 'The Rov said it' flag waving. I reject that kind of behavior summarily. My intention was solely to balance the discussion started by Gil.