Harry Maryles' post about mixed weddings is a must read. It's a summary of a speech by Rabbi Rakeffet about the false religiosity inherent in the separate seating scheme.
Put simply, I agree. Let's be serious: in the 21st century most of us mix with the opposite sex for much of the day and seeing them at weddings is not going to bring out thoughts that aren't already there the rest of the day. This argument, as noted in the post, is precisely the argument of the Levush.
But on a broader scale, how do the changes in society affect the whole halachic framework of the mixing of the sexes? The Shulchan Aruch (via Frumteens) says:
"A person must stay far, far away from the women, and it is prohibited to signal with your hand, to hint with your eyes, to any prohibited women. It is furthermore prohibited to laugh together with them and to be frivolous in her presence, or to watch her beauty. Even to smell her perfume is forbidden...."
The Gemara says it's forbidden to say hello to a married woman, even through her husband. There's also the big requirement for married women to cover their hair. Do all these halachos apply in the same force today, given how society has changed?
One point Rabbi Rakeffet makes in his speech is the societal aspect of tzniyus. The Rambam holds that a woman must wear a "radid" which is some form of veil. Today no woman in a western country would do that. Why? Because society no longer considers that modest; it considers it crazy.
So if how society views modesty changed halacha in that case, does it change it in the other cases above? When men and women don't interact, then those mild interactions could have some meaning. But today almost everyone spends time with members of the opposite sex. Maybe other people are different, but I feel nothing when I say hello to my married friends' wives. It's the same with my female friends. Maybe I'm unique (or dishonest) but I doubt it.
Moreover, the prohibitions overlook the positives of a male-female relationship. Having a friend of the opposite gender allows a person to learn how the other gender thinks, acts, feel, etc. This valuable information comes in handy during dating and marriage. Moreover, in the past marriages were less compassionate (by that I mean they were less about forming meaningful relationships and more about fulfilling duties), so this type of information was not as necessary.
So while in the past the calculus of cost vs. benefit might have tilted in favor of prohibiting male-female contact, perhaps today, given the changes in what these interactions mean and how important male-female friendships are, perhaps they should no longer be prohibited.
These aren't halachic arguments, but more arguments of policy. I haven't looked at the sources, so I can't really give a halachic argument. But halahca recognizes changes based on social mores in some cases. I'm just wondering how far it goes.