The Buon Me Thuot region of Vietnam is famous for its potent java brew. But when young Vietnamese talk about ca phe den mo (literally, "dim light coffee"), they're not referring to a special blend. "Dark coffees" — cafes lit so dimly it's often impossible to see within them — are a Vietnamese institution, and like American drive-in theaters or the love hotels of Japan, they provide a crucial service for couples: public access to privacy.

Dark coffees can be found all over the city. The greatest concentration is in the Thanh Da area on the western outskirts — Saigon's "dim light district." The Mekong runs through it, providing a romantic perch for many of the cafes. Others are tucked into inland side streets, away from city lights and traffic. At minimum, a dark coffee is dim enough so that you won't be recognized; others are pitch black, and their navigation requires the aid of a flashlight-carrying waiter. All seats face the same direction, so your view is limited to the backs of other couples. Each set of chairs is separated by a divider, usually a vine-entwined screen or a solid partition. Music masks the smacking of lips and emittance of sighs. It's all atmosphere — at twice the price of a normal café. People don't come to drink, and waiters don't cruise around asking people for second orders, so the house gets only one chance to make a sale. And once people are settled inside, they're usually in no hurry to leave.

It is Vietnam's stringent sexual taboos that drive couples to dark coffees. Most Vietnamese men will not marry a woman who is not a virgin, sometimes even if she's only slept with him. An unchaperoned visit to a man's apartment or a hotel room is viewed as an open invitation for intercourse or sexual assault. And even if a woman leaves such a situation with her virginity intact, her reputation will be sullied. "Of course if they are in a room together, they will do it," says Bui Ngo Duong, a 30-year-old teacher. "Why wouldn't they? Whether or not they did it, we have to assume that they did."

Dark coffees allow women to explore their sexuality without destroying their personal lives, providing a safe environment and an alibi. The cafe's level of illumination provides a crucial check on libidinal urges while allowing couples to indulge in what feels comfortable — the higher the lumen, the safer the hymen. When a woman chooses a dim café, she's indicating that kissing and cuddling is a possibility; a darker selection usually means that under-the-shirt fondling is on the menu, and so on. The café acts as chaperone.

But not all dark coffees preclude intercourse. Some have reclining seats that are almost bedlike and partitions so sturdy they are almost rooms. Here is where denial becomes paramount. After leaving a hotel room or apartment together, a couple is assumed to have screwed, and the woman thus tarnished. After leaving a coffee shop, they can still deny it. For unmarried couples, abstinence is not as important as appearance.

Cost is also an issue. To an American, spending $4 for a room rather than $1 at a café would be well worth it, but a Vietnamese citizen earning $80 a month has a different perspective. Hotels are off-limits to unmarried couples, and even monied Vietnamese view them as "a waste of money," in the words of Mr. Dung, a relatively well-off dark coffee regular who refused to give his first name in order to protect his girlfriend.

Although they're highly popular, dark coffees are hardly accepted. It doesn't help that although the majority of the cafés are couples-only, the term also refers to places where a single man can have a "girlfriend" supplied to him. Even women with long-term boyfriends almost never admit going. Some refuse to go at all. Cam Bui Ly, a writer who has studied in Singapore, explains: "I know my friends go, but we can never talk about it." Some of them complain that their boyfriends are xao qua (horrible) for even suggesting it. Men also disapprove: Nguyen Minh Tan, a recent university graduate, will not bring his freshman girlfriend to a dark coffee because he is afraid of "making her mind dirty." They may go after she graduates, however. "Then, she enters real life," he explains.