To my knowledge, there isn't a hard rule that says there must be gender segregation in the langar hall. Some gurdwaras have "co-ed" sections along with their "segregated" sections.
The main reason that I can think of is modesty. When we are in the kirtan hall, we sit, stand, and matha tek many times. Sometimes we do this in crowded conditions. Sometimes when doing this, we stumble or lose our balance and bump in to each other.
We do not go through these body movments in the langar hall, at work, on the subway, in the elevator, or any other common situation that places us in crowded, mixed company.
Personally, I really like the idea of having a part of gurdwara available for familes that want to sit together while worshipping, but for as long as I am single, I plan on sitting amongst my fellow Kaurs while in gurdwara. I really don't want to be worried about whether the person sitting behind me is looking at my backside with intentions that are...not gurmat. Plus, if I'm clumsy enough to stumble in to a woman while climbing up from a matha tek, it is quickly forgiven and forgotten. If I stumble in to a man while doing the same, it could become rumor fodder for the next few weeks. :crazy: No thanks.