What trouble can a furtive affair bring to the lovers? What misery can they cause their official partners? How do two ordinary cowpokes handle the kind of sexual fever they were taught to think is unnatural, and that could get them killed? Movies, searching for good guys and bad guys, rarely achieve a balance of sympathies between the ecstasy of sexual attraction and the pain of the people betrayed. Ang Lee’s film of the Annie Proulx story brilliantly articulates the unspoken; it gets the moviegoer’s pulse in synch with the beating of two — make that four — ravaged hearts. Unsentimental, unforgettable.