Georgetown's Trinity Theatre opened its 1980-81 season last week with a comedy about "well-to-do" married couples. Considering the Georgetown demographics, familiarity is at least one of the best features Trinity has to offer with the current production of W. Somerset Maugham's "The Constant Wife."

Maugham's three-act play is an ambitious work for a community theater to undertake, for commanding audience's attention for 2 1/2 hours is no cinch. But the Trinity production, although sometimes drawing a yawn here and there, showcases several local actresses whose excellent character studies keep the show interesting despite the dragging pace.

Christine Abel defines charm in the title role of Constance Middleton, the poised and polished doctor's wife who cannot be dissuaded from being the perfect mate to an unfaithful husband. Abel, who works here for the World Bank, speaks throughout the play ever-so-genteelly and with the sweetest in her voice.

"She's eating well, dressing well. She's even losing weight. What woman wouldn't be happy?" Ruth Henoch as Constance's mother (a 1920s Marabel Morgan) has some of the best lines in the play and carries them off splendidly, with a mild-mannered feistiness. Henoch teaches English composition at American University. Mary Ruberry, an arts teacher, is equally charming and effective as Constance's brooding sister.

The Trinity cast, particularly Abel, loses credibility, however, when moving about the set -- a pleasant design by Gerry Stringer. Abel saunters gracefully, to a fault. Her white-glove hands are either always clasped or casually extended like a model-guide at an appliance showcase. Also, her to-and-fro movement on stage is often dizzying.

Male characters in this production are dull in contrast to Abel, Henoch and Ruberry. Steve Nelson, who plays Constance's old college sweetheart, decides in the second act to confess his love: "For just this one moment I'm going to let myself go." Nelson, and Peter Avery as Constance's husband, never quite do. However, they have two more weekends to try.

"The Constant Wife" continues at Trinity Theatre, 3514 O St. NW, weekends through Oct. 25. For more information call 965-4680.