It takes a lot of food to satisfy a 336-pound man. Particularly a 336-pound man who's trying to double his weight. And yet Jorge Rivera, sitting in a midtown Manhattan diner, is almost demure when he orders his dinner: one standard-size cheeseburger, Philly-style; one order of sweet-potato fries; one Diet Coke. (It's the Diet Coke, incidentally, that prompts a priceless double-take from the seen-it-all waiter.) After he's finished, Rivera settles back in the booth and wipes his goatee. The Formica table shifts beneath his elbows, and his belly, clothed in a Mets T-shirt (size: XXXXL), is almost flush with the table's edge.

Eight years ago, when he was 19, Rivera dated a woman who wanted him to add 20 pounds to his six-foot, 280-pound frame. At restaurant buffets, she would cram plate after plate with fried fish, mac-and-cheese, gravy-soaked meats, and multiple slices of pie and cake. He ate dutifully—first his plate, then hers—until his stomach emerged from beneath his shirt. Then they would go home and have sex. Or they might first get in bed, naked, while she sat on his mighty thighs, feeding him slices of pizza or forkfuls of chocolate-mousse cake and rubbing his bulging belly until they couldn't hold off any longer. Within three months, Rivera had packed on a heart-stopping 50 pounds—and discovered the erotic charge of watching his waist expand under his lover's approving gaze. "It's very exciting to have her get turned on by the fat," Rivera says. "It's a simple equation: more food, more fat, more sex."

Dessert arrives—a wedge of German-chocolate cake—and when that's gone, Rivera lifts his fleshy stomach and proudly flops several inches of it onto the table. On his way home from the diner, he plans to stop at McDonald's for two double cheeseburgers and an order of 10 Chicken McNuggets.

Rivera is a "gainer": an enthusiastic participant in the expanding subculture of feederism, in which fat fetishists get aroused by "growing" themselves or their romantic partners, consuming their way to morbid obesity. Feederism has been around at least as long as cupcakes, but the fetish is rapidly gaining recognition (and fans) through Internet exposure. It's an extreme variation on fat admiration: Rather than merely preferring large partners, "encouragers" (who cheer on "gainers") and "feeders" (who serve piles of food to "feedees") also get off on embodying the public's fear of fat, and in knowing their sexual taste strikes people as deviant. "It's like being in a club with a few exclusive members," Rivera says. "Society never did anything for me, so why should I try to be what society wants?"

Although, in a sense, he gives society exactly what it wants. While the calorie counters at nearby tables request their salad dressing on the side, Rivera gloats at having trouble squeezing into a booth. And whenever he overhears whispers from other customers, he signals to his waiter and orders more.