"We're all here to get conjugally matrimonified!" exults one of the altar-bound privateers in the finale of The Pirates of Penzance—the scene wherein the show's young lovers, honest orphan Frederic and his sweet dippy Mabel, finally get hitched. Yet with all the sugary matings they cooked up in their careers, even Gilbert and Sullivan might blink at the latest pairing they confected.

He is 26, sweet of face, gentle of disposition, and diligent as all get-out. She is 29, sexy and bouncy, with a singing voice to make a pirate king swoon. But the really improbable part is that Robby Benson and Karla DeVito met last year while playing Frederic and Mabel in the Broadway production of Pirates, married last July and have lived happily ever after—which is, so far, only three months. "In the back of your mind you have to wonder, 'Is this real or an extension of the stage fantasy?' " Karla recalls thinking during their courtship. "It's very confusing. But now I'm sure this is real."

In a business that encourages steadiness the way horse racing encourages thrift, both Benson and DeVito are known as hardworking and nice—and busy. This week Robby, who also sings and writes songs and scripts, stars in the CBS movie Two of a Kind as a retarded lad who tries to imbue his lonely grandfather (George Burns) with his own zest for life. "Benson's like Montgomery Clift—you can read all over his face where he's going," says director Roger Young. "He's the most sensitive person I've worked with." Karla, who also acts and writes songs, has—besides a small role in Kind—a much-praised first album, Is This a Cool World or What? "It's not often you find a lighthearted sex symbol with a brain," says a pal, rock manager Billie Best. "Karla's one of those rare people who is a tremendous talent and a real person as well."

Today Benson and DeVito are "each other's cheerleaders," Robby says. But when they met, on the Pirates set, Karla couldn't have cared less about romance: "I was going on a nice, smooth course, and I wasn't expecting to fall madly in love with anyone." Robby, as is his style, was being a bit of a loner—so much so that the show's music conductor, Dan Berlinghoff, decided to take him to lunch with cast members, one at a time. The first outing was the last. Berlinghoff paired Benson with DeVito, and they found they'd admired each other all along. "I've been working since I was a baby," Robby says, "and I've never been blown away by a performance, but when I heard Karla sing I was impressed to the point of being embarrassed." "I liked him totally," Karla recalls. "He was so creative and a lovely, humble person. I'd thought of him as very serious. To find that he also juggled plates and walked into walls was fun."

In two months Benson decided he wanted her as his real-life bride. But of course, no G&S romance is worth its salt without a separation. Karla had not had time to sort out her feelings when she left the show to go on a prearranged three-month record promo tour. Though she scribbled "Faith" on his dressing-room mirror, Benson was "devastated," he says. Last January, when he left the show, he asked her to come live in his San Fernando Valley house. She did. They wed in her hometown, Mokena, Ill., on July 11. When they sang a duet he'd written called/ Believe in Fate, "There wasn't a dry eye in the room," Karla remembers.

Fate intervened in their plans for a Pirates road tour together after their Hawaiian honeymoon. Robby landed the lead in Running Brave, an upcoming movie about Billy Mills, the Sioux Marine lieutenant who won the 10,000-meter run in the 1964 Olympics. Though Benson had to quit Pirates, get his locks cut to Marine specs and begin filming right away in Edmonton, Canada, he relished the chance to portray Mills' against-all-odds triumph: "My parents taught me that anything you believe in, you can do."

Benson was born Robin Segal in Dallas, where his screenwriter father, Jerry, was then in the cotton business. The family, including his actress mother, Ann, were all theater buffs. "My earliest memories are of listening to Mother and Dad work with other writers, actors and musicians," says Robby. "There was never any doubt what I wanted to do." At 5, he debuted in a Michigan summer stock production of The King and I with his mom. At 8, he hired a manager. When he was 10, his parents changed his name to his mother's maiden name, believing it to be a better showbiz handle than Segal.

The family moved to New York in 1962, when Jerry Segal's writing career blossomed. At 15, Robby was excelling on the Lincoln Square Academy's track team by day, acting in Search for Tomorrow in the afternoon and appearing on Broadway in The Rothschilds at night. He also made his first movie—Jory, a Western. Though he was so busy by his senior year that he had to do his studies by correspondence, he was class valedictorian.

To pass the time between camera calls, Robby and his father liked to make up short stories, and—presto!—by 19 Robby had sold a major script. It was One on One, the 1977 sleeper about a school basketball star; Benson was not only co-writer (with his dad) but also played the lead. Since then he's been a pro skater in Ice Castles (for which he worked out with the N.Y. Islanders and fractured a hip), Jack Lemmon's anguished son in Tribute, and recently a Hasidic Jew in The Chosen.

Karla was one of four children born to Vivienne and Sebastian DeVito, a construction company owner. After her father died when Karla was 8, her mother went to work, eventually becoming a Woolco store department manager. Karla studied acting at Loyola University, then dropped out to join the national company of Godspell. Later she moved to New York, switched to rock singing, and spent 18 months with Meat Loaf before joining Pirates on Broadway as understudy to the first Mabel, Linda Ronstadt. Now Karla is composing a rock opera called Bloody Bess (it's about a lady pirate) and taking Benson lessons: "Robby's persistence is so great. It's something I hope I can pick up from him. I've always approached work from the raw energy point of view—I go out and do it without a plan."

Robby, who once lived with actress Merilee Magnuson, changed Karla's who-needs-it view of matrimony. "He was the first person I met who spoke of marriage in a real sense," she says. "He had such a lovely way of talking about the future. Maybe because of my father's death I'd never thought about the future. I realized the rug could be pulled out from under you anytime. This relationship is going to be the anchor of my life." What does she do for him? "She reminds me of what's important," Robby says. "And she makes me laugh."

She is also working a little feminism on him. With a mother who has had a successful career giving Merrill Lynch seminars on money management for women, and a sister who is a fashion designer, Robby has always taken female accomplishments for granted. "So when Karla buys Ms. magazine it makes me smile. I guess I don't realize all that women have to go through." Children? As soon as possible, please. "You know how it is being a woman," Karla says. "There's this paranoia: 'Well I'm 29 now and I've got so many good years left with my ovaries.' " "I would like to take six months off and be a househusband," Robby declares firmly. "I can't think of anything better than following Karla around with a baby."