On her wedding night, a young woman goes into the bathroom to put on a lovely white negligee. Still wearing the flowered headband of her wedding veil, she tiptoes out into the bedroom. There she sees her husband dressed in a long, red, flannel nightshirt, black socks and a sleeping cap. He wears a grin that would scare a ghoul.

Some might say that that honeymoon was over before it began.

Others might recognize this as one of the ha-ha-ha opening scenes from “I Do! I Do!,” a 1966 musical that’s easy to both love and hate. Now playing at Acadia Repertory Theatre in Somesville, “I Do! I Do!” follows two newlyweds from marriage to kids to midlife crises to affairs to the empty nest to retirement. It’s funny, witty, romantic and sentimental. So it’s bound to be popular. It’s also dated. So it’s bound to be maddening to some, and mostly silly to others.

But as the season opener at Acadia Repertory Theatre in Somesville, “I Do! I Do!” is a perfect offering. It’s lighthearted enough to be enjoyable to a wide range of audience members, and insightful enough to make conventional couples nod their heads in recognition of some universal situations of relationships. What makes it apropos for summer theater is that it’s effortless to watch, and everything turns out peachy. Even if you aren’t on vacation, “I Do! I Do!” is the type of upbeat entertainment that’s a blast to see from time to time.

Plus, the music is lovely. A collaboration between Tom Jones and Harvey Schmidt, who also wrote “The Fantasticks,” there are many memorable songs from this one, the most famous of which is “My Cup Runneth Over.” But equally enjoyable are “Nobody’s Perfect,” about the foibles only a spouse can know, and “Love Isn’t Everything,” about the way life changes when children come into a marriage.

The two-person cast of Siobahn O’Neill, as Agnes, and Micheal Sears, as Michael, have their strongest moments in the first half of the show, when the characters are closest to the actors’ actual ages, and, in fact, both the music and writing are also more interesting than in the second half.

Throughout the show, however, O’Neill is a real sweetheart — as a young bride, young wife, young mother. Her “Flaming Agnes” number, in which she cuts loose with her wilder side, has the charming combination of new-found release and delight. O’Neill has a particularly pleasant voice — friendly and clear and easy to listen to. And it’s a good thing, too, because she has to sing some of the most unfortunately retrograde lyrics, such as “To be a woman means being lonely. That’s why a woman is only alive when in love.”

Sears is full of energy and has many funny lines. Too bad he never really shows what makes Michael so lovable to Agnes. Generally, he plays up the “little boy” side of his character and it wears thin quickly. Just because the show takes place in an earlier era (1890-1940 is the span of the show) doesn’t mean that Michael has to be any less sophisticated even if some of his outlooks are quite unappealing these days.

Pianist Robert Bahr does a fine job at providing a rich accompaniment, and costumer Rae Johnson provides some breathtakingly elegant dresses and suits.

Director Ken Stack knows that this is good summer fare and has the actors dancing around in old-time routines (choreographed by Judy Savage) and making the most of the ba-dum-bum one-liners. Some of the ideas are real dinosaurs, but Stack thankfully keeps things straightforward and clean. Parts of the show drag, however, especially because Stack has all but one of the burdensome costumes changes take place offstage while the audience sits in the dark — sometimes even without music.

Plus, Stack is a purist when it comes to scripts and insists on leaving scenes in that might have been thankfully cut. There’s really no reason for this show to go over two hours. In fact, it’s such a collage of musical numbers and sitcom scenes that speeding up the timing or trimming the script might have given it an extra boost.

“I Do! I Do!” will be performed 8:15 p.m. Tuesday-Sunday through July 9 at Acadia Repertory Theatre in Somesville. For information, call 244-7260.