To those posters saying "it's just a convenient way of ensuring equal numbers of each type of dish" can I introduce you to the concept of "stereotype threat"?
Stereotype threat is a well documented psychological phenomenon whereby when you live in a culture which holds certain stereotypes about certain groups of people ("girls aren't good at maths", "boys don't like reading much and have to be forced to do it", "black students are better at sport than academic subjects"), being reminded of that stereotype can reduce your performance in tests. It's immensely powerful - you can reproduce it entirely artificially. There have been experiments done where classes of college students are given the entirely false "information" at the beginning of a class that "people with brown eyes are better at algebra than people with blue eyes", then given an algebra test at the end of the class, the blue-eyed students perform worse than the brown-eyed students - being told you're no good at something is enough to depress your performance even if there's no basis in truth.
And with real-world stereotype threat, we're so immersed in this culture that you don't need to say "girls aren't good at maths" to the group, you just have bring "being a girl" to the front of their minds. So, for instance, with job tests, if you do the "sexual and racial diversity questionnaire" before the test, women's and BEM candidates' test scores drop off relative to groups where the questionnaire is administered after the tests. (Cordelia Fine's Delusions of Gender has a good chapter on this).
This is why (and I believe this is taught in teacher training courses these days) it is considered good educational practice not to split groups into "boys over here, girls over there" as a quick (and lazy) way of dividing a class in two (unless of course you have an actual reason, like wanting to deliver sex ed without having the boys embarrassed when the girls giggle about wet dreams, and the girls embarrassed when the boys giggle about periods).