In the flick <How I met your mother>. When Marshall had a brunch himself only or with another man, the waiter felt so strange and his friends laughed at him about this.
Is it a special meal? Is it commonly for 2 people, a male and female?
I think that Ted and Robin are just making fun of Marshall going to a restaurant with a man rather than with a girl.
It wouldn´t have mattered whether it was brunch or dinner or whatever. It was just because his friends are suggesting he is dating a man, so he might be homosexual.