OPERA IS REPLETE with courtesans," noted the Naughty Marietta of Bethesda, who then sang "Love is worth more than all the riches in the world." She stood by our table, looked deeply into my date's eyes, her hand resting on his shoulder. For a while there, it looked like they'd be leaving together after she'd finished her aria and he'd polished off his piccata.

Shirley Markham, the coloratura in question, sings well and regularly at Asti Roseto, an Italian restaurant that offers a romantic outing for opera fans on Thursday through Sunday nights. "Imagine Rigoletto with your rigatoni, or Verdi with your veal," says owner Frank Lantella. We had Mozart with meatballs and Der Ring des Nibelungen with our didilini.

Besides the heavy stuff, there are operetta and show tunes, sung by such pairs as Margaret Stricklet and Stanley Dunn, a duo on the order of Nelson Eddy and Jeanette MacDonald. Dunn stands under a wrought-iron trellis festooned with plastic grapevines and pines for Stricklett, his arms outstretched, "Till There Was Yooo."

Next, he swings into a rousing rendition of "76 Trombones," and while marching toward the grand piano nearly bowls over a waiter. It's intimate. Cabaret singers aren't like Muzak. You've got to respond when they come up to your table. Put down your fork, stop chewing, look interested and applaud.

The same goes for the performers, who have to work extra hard to be heard over the chewing. "You have to be very concentrated," says Markham. "People think nothing of talking to you in the middle of a lyric."

Well, Asti Roseto is a friendly place. The owner's a loquacious host who likes to tell about how his hometown of Roseto, Italy, has a sister city, Roseto, Pennsylvania, near Allentown, where they get fewer heart attacks than anywhere else in America. The restaurant is named for both. If you don't run into Frank Lantella, the story's on the menu.

The restaurant is a modest one, hospitable, if hot. Entrees average from $5.95 for pasta to $12 for veal, and a bottle of wine about $9.

The menu is expressive, offering such tidbits as tortellini in brodo ("pasta filled with meat, shaped into little hats, in chicken broth") or patos alla chiana, an entree with a history lesson -- its homemade egg noodles were cut with a guitar string in the olden days.

There are carnations on the table, and tiles from many lands on the floor. The tile's okay, but you can never see your friends over the flower arrangement. We asked a bus girl with a pitcher to take ours away. But she didn't speak English, so she watered it. ASTI ROSETO -- At 7940 Wisconsin Avenue, Bethesda. Italian lessons free with the purchase of dinner, Monday to Wednesday; opera, Thursday to Sunday. 652-1300.