All Diane Keaton and William H. Macy need to justify an assignation, cheating on Richard Gere and Susan Sarandon, is a cheap motel, a bucket of chicken, a six pack, cable porn and no “Bible” in the room.

Cheaters have to have standards, after all — ground rules.

Gere and Sarandon, playing their spouses, met under other circumstances and started their own affair. And that was months ago.

Is it any wonder that these two couples, who don’t know each other as couples, raised a daughter who dreams of fairytale love that lasts forever and a son who hurls himself between his beloved and the bouquet she is certain to catch rather than face up to the Big Question? His and her parents are still wrestling with that, decades into their marriages.

“Maybe I Do” — the question mark is implied — is a beautifully cast and performed trifle of a rom-com. It’s about marriage and commitment, boredom and unhappiness, straying and guilt. But mainly it’s about a nagging doubt that the blush of new romance hides, but which might never go away.

“Are we living our best lives?”

The AARP-qualified leads — two Oscar winners among them — deliver snorts and cackles from the bitter, biting and cynical exchanges, rejoinders and petty humiliations they lightly fling at one another.

And then the young people these two unwittingly-connected-by-infidelity couples gave birth to, Michelle (Emma Roberts) and Allen (Luke Bracey) — quarreling, splitting up and yet still hoping for a Hail Mary — resurrect the face-flushing warmth of a dream worth clinging to, idealized love shared for a lifetime.

Writer-director Michael Jacobs, adapting his own stage play, has produced a clockwork rom-com, ticking over with a precision born of dialogue, situations and blocking polished on the stage.

It’s too on-the-nose, too tidy and entirely lacking an edge. But this cast delivering that dialogue? That’s worth checking out.

Compassionate Grace (Keaton) meets weepy Sam (Macy) at a showing of a melancholy subtitled Scandinavian romance about old age.

“I can’t satisfy my wife,” is his sad admission, when the fried chicken and sixpack fail to set the mood. They spend a night walking and talking instead.

The bloom has gone off the rose of the affair Howard and Monica (Gere and Sarandon) started. They’ve checked into a much nicer hotel. She’s in a silk robe and ready for some action.

“I’m naked under this,” she purrs.

“I’ll take that into consideration,” he grumps.

Her “I exist, and you hurt me” leads to a seething threat of “killing you” just when Howard least expects it.

“Nobody has to kill anyone. Time is doing a fine job of that.”

And just as these mismatched, timeworn couples are taking stock of the “taking stock” that made them want to stray, Monica and Sam’s son Allen dives for that bridal bouquet and humiliates “perfect” Michelle, daughter of Grace and Howard.

“It was the most awful moment in the whole history of women being stuck with you idiots!”

Can any of these relationships be saved? Should they?

Few actors and even fewer actresses get offered decent parts their 60th birthdays, and judging from the many feeble Keaton vehicles in particular that cross my radar, it’s a relief when they land material that is worthy of their experience and reputation.

A certain all-star Super Bowl comedy opening shortly comes to mind when one thinks of the movies that are unworthy of their legendary talents.

“Maybe I Do” is cute enough, despite its sanitized-for-your-protection limitations. Everything is boiled down to love in a vacuum here. Two affluent-enough families with two beautiful 30ish offspring ready to “jump off a cliff” together — or not — and there’s no talk at all of the things that complicate romance — careers, work, money, comfort and status.

It’s a film that can’t hide its theatricality. The dialogue is pat, neatly-spaced and separated for this or that actor’s speech or joke/punchline exchanges. None of this roiling, messy bickering and talking over one another that sounds and plays like real life.

But allowing time for Sarandon’s testy Monica to take a shot at a young, pretty hotel clerk who dares to “Ma’am” her, giving Gere a grace-note moment in a diner when he and a 40ish waitress can look on young lovers in a booth and admit “It’s not our world, any more,” has value.

That makes for a bittersweet, wistful rom-com that may not be all that it might have been, but isn’t all that bad as it is.

Rating: PG-13 for sexually suggestive material and brief strong language

Cast: Diane Keaton, Susan Sarandon, Richard Gere, William H. Macy, Luke Bracey and Emma Roberts.

Credits: Scripted and directed by Michael Jacobs, adapted from his play. A Vertical release.

Running time: 1:35