Everything about Seth Rogan's character in Knocked Up is meant to portray a loser: his hair, his face, his weight, his clothes, his friends, his lack of ambition, his pot smoking, his apartment, all of it. Everything about Katherine Hegel's character is meant to portray a winner: her hair, her face, her figure, her clothes, her family, her ambition, her success, her lifestyle, all of it. Apatow's casting is no accident.
Clearly: the loser (male) wins the winner (female). Yes, this makes the film sexist. I have other issues with Apatow's scripts also, but this portrait of the male/female relationship annoys me the most.
BTW, Elizabeth Bennett is not in the same English social class as Mr. Darcey, but she's not a loser. She's brilliant, clever, and socially capable. All that happens to figure largely into the entire point the novel is making. If you think she's a loser who got the rich guy, you really need to re-read the novel or rethink your position.
Glad to get all this off my chest. Carry on!