Daniel Mendelsohn gets it right in the New York Review of Books:

The real achievement of Brokeback Mountain is not that it tells a universal love story that happens to have gay characters in it, but that it tells a distinctively gay story that happens to be so well told that any feeling person can be moved by it. If you insist, as so many have, that the story of Jack and Ennis is OK to watch and sympathize with because they're not really homosexual—that they're more like the heart of America than like "gay people"—you're pushing them back into the closet whose narrow and suffocating confines Ang Lee and his collaborators have so beautifully and harrowingly exposed.