LIPS TOGETHER, TEETH APARTComedy by Terrence McNally, directed by John Tillinger. At Manhattan Theatre Club. THE sun shines bright on the Fire Island summerfolk gathered for a Fourth of July weekend in "Lips Together, Teeth Apart." The psychological horizon is more clouded. Personal and marital insecurities underlie the laughter and occasional pathos of Terrence McNally's moderately entertaining domestic comedy. In the manner of such genre pieces, things change and yet remain substantially the same for these middle-aged middle Americans. John and Chloe Haddock (Anthony Heald and Christine Baranski) are close friends and house guests of Sam and Sally Truman (Nathan Lane and Swoosie Kurtz). Sally has inherited the hospitable cottage from her late brother, whose death from AIDS she describes at a moving point in the play. References to homosexual neighbors further define the context of the holiday encounter. "Lips Together, Teeth Apart" focuses mainly on the interaction between and among the weekending couples and on the implications of a momentary infidelity. The characters form a recognizable group. Flaky Chloe is the compulsive provider, always at the ready with offers of meals, drinks, and incidental nourishment - spiced with French phrases. Husband John harbors a health secret of which the Trumans become gradually aware. Sam Truman is the odd man out, a New Jersey building contractor ill at ease with in tellectual concepts and fancy foods. Sam's wife Sally is a romantic whose discovery of a drowning climaxes the second of the play's three acts. Otherwise, Mr. McNally's weekenders pass the time in familiar pursuits. Sally paints. John does a crossword puzzle and flies a kite. Sam putters. Prompted by a sudden urge, they break spontaneously into "America, the Beautiful." They also try to remember the refrain from "There's No Business Like Show Business." At one point, John and Sam even scuffle in McNally's version of a typical July 4th weekend. The rueful comedy is smoothly acted under John Tillinger's sympathetic direction. Sharp and delicate characterizations exist within the give and take of a superb ensemble: Ms. Baranski ever the delectable comedian, Ms. Kurtz sensitively attuned to Sally's introspections, Mr. Heald a deceptively self-contained intellectual, and Mr. Lane a literal-minded primitive frequently out of his depth. John Lee Beatty designed a handsome, breeze-swept beach house. Jan Greenwood costumed and Ken Billington lighted the summery production. The title, incidentally, refers to a cautionary phrase for people who grind their teeth when falling asleep.