Someone watching this summer's touring theater scene might notice what seems to be a strange phenomenon of married couples (and ex-couples) acting out their romances on stage, what with the Taylor-Burton "Private Lives" event, and now, "I Do! I Do!" starring Lucie Arnaz and Laurence Luckinbill at the Warner Theatre.

Based on Jan de Hartog's play "The Fourposter" and originally staged by David Merrick and Gower Champion in 1966, "I Do! I Do!" features book and lyrics by Tom Jones and music by Harvey Schmidt, both of "The Fantasticks" fame. This musical, about 50 years of marriage beginning at the turn of the century, has the same kind of aggressively simple, hummable score as Jones and Schmidt's first hit, and features at least one dentist's office standard in "My Cup Runneth Over."

Arnaz and Luckinbill's latest outing is briskly paced by director Lucia Victor, especially when compared with last year's interminable "They're Playing Our Song," and overcomes some annoying sound problems.

The musical is buoyant, pre-sexual revolution entertainment, harmlessly flaunting its innocent belief in the double standard. (She: "At first there'll be years and years of crying and diapers . . . " He: "Oh, I don't mind, I'll find something to do . . . ").

Neither Arnaz nor Luckinbill is a born singer, and that would appear to be a detriment in a two-character musical with little spoken material. Arnaz can carry a tune, but her voice, though quavery and pretty, is frail and frequently misses crucial notes. And Luckinbill has worse luck still, although he tries gamely and has a fine comic way with the words. Somehow the two make up for each other's vocal inadequacies when singing together, and during Act 2, their "post-middle age" years, their voices are actually in their favor.

Both actors have more success with the comedy, and their timing is terrific together. Arnaz has some particularly funny bits, especially while expressing first-time terror on her honeymoon night and while expertly riding a tricycle. She seems to have nicked quite a bit of schtick from her famous mother, Lucille Ball. Arnaz has absorbed "Lucy's" bawling and mugging and adapted it perfectly for herself. She also seems to be developing as an actress, and is appealing in her portrait of Agnes' middle and late years, without relying on hackneyed "old lady" gags.

Luckinbill is also quite funny as Michael, even succeeding in making himself believably cute as a boyishly beaming teen-age groom on his honeymoon. Both actors age effectively onstage, although Arnaz's overdone eye makeup makes her appear more ghoulish than glamorous.

The show hits its stride when the couple has its first spat in "Nobody's Perfect." Both are delightful in a matched set of solos. Luckinbill takes obvious delight in singing "A Well-Known Fact" (that men grow more attractive as they age); and Arnaz almost outdoes him, countering with a flamboyant "Flaming Agnes," dressed in a blazing red kimono.

Oliver Smith's simple set, basically an oversized, stylized bedroom suite, with prominently featured four-poster bed, is attractive and fits comfortably into the Warner's proscenium. Plenty of amusing distractions fly in and out, espcially in a snappy number when the two children arrive, with all the attendant toys and laundry.

This production's only real problems seemed to be technical in nature. Because of the actors' weak voices, mikes are necessary, but the problems at the Warner were inexcusable. During the first half, Jones' witty words were often lost or muffled if a character turned or moved, and in the second half, the voices were so strangely echoed and overamplified, they might have been singing in the Super Bowl.

I DO! I DO! Book and lyrics by Tom Jones, music by Harvey Schmidt. Directed by Lucia Victor. Music director, Gordon O. Brown; set design, Oliver Smith; lighting, Ray Dooley; costumes, Michael Bottari and Ronald Case. With Lucie Arnaz and Laurence Luckinbill. At the Warner Theatre through tomorrow.