Benjamin lives in a world of suburban castles with wide lawns and pools, organization-man husbands, and drunk wives, the logical culmination of the heterosexist myth, a glimpse into his future, a glimpse into the future we were all told that we should long for.
He spends most of the movie trapped, staring mutely from behind fish tanks, wet-suit visors, wide shots angled to suggest enclosed space, and Mrs. Robinson's legs shaped into a triangular dragnet. There is no escape from his Stepford world, not even among the hippies. When he goes to Berkeley, he finds no shaggy-haired, tie-died counterculture, just straights with textbooks. Roger Ebert says that he is "utterly unaware of his generation."
The adults seem to notice, and obsessively try to prod him into heterosexual practice, always suggesting that he "call a girl." When Mrs. Robinson first approaches him, he rushes horrified down the stairs, where Mr. Robinson sits him down and has a heart-to-heart: "You should be having fun with girls!" Benjamin protests that he is not interested in girls.
Because the bus is taking them right back to the suburbs, where they'll buy a house, and Benjamin will sell plastics, and Elaine will sign up for charity drives, and in twenty years he'll be a workaholic, and she'll be an alcoholic. "The one" inevitably becomes Mrs. Robinson. Heterosexual love provides no escape. They are trapped.
See also: The Graduate Revisited