“Significant Other” is a slick, well-made, funny-sad new Broadway comedy, the kind that doesn’t often get a first-rate commercial production these days. At its soft heart, however, the play is really a 21st century theater throwback to that old song that cried, “Those wedding bells are breaking up that old gang of mine.”

That standard was written in 1929, but, clearly, marriage can still be a disruptive force on some buddy units of any demographic. For young playwright Joshua Harmon, the gang comprises three women friends in their late 20s and Jordan Berman, the witty, gay sidekick of so many passe sitcoms and Hollywood rom-coms.

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Only in “Significant Other,” which comes to Broadway after a successful Off-Broadway run, Jordan is not the sidekick. He is the main character. He is still the familiar, sardonic, fun-loving and just a little self-loathing character who has quipped from the sidelines for decades. Here, however, he is the one who feels increasingly abandoned as he attends everyone else’s bachelorette parties and recites touching poetry at his best friends’ weddings.

Jordan, played with deft comic timing by Gideon Glick, is also a romantic — make that an obsessive — whose crush on a tall, handsome co-worker may suggest to some theatergoers a pathology beyond mere boyish adorableness. For a grown man, he falls in love too hard and too easily. We search in vain for an interesting internal life that might match his quick humor.

But even in touching scenes with his grandmother — layered with exquisite drollery and sorrow by Barbara Barrie — Jordan is unable to get beyond his unraveling bond with his girlfriends, his empty job in advertising and his desperate need to find a life companion.

Director Trip Cullman draws smart, entertaining performances from Lindsay Mendez, Rebecca Naomi Jones and Sas Goldberg, who is saddled with some jarring observations about body functions that don’t ring true. Mark Wendland’s handsome set is an ingenious modular construction that piles up contrasting scenes behind screens, and Kaye Voyce’s costumes grasp the shocking transformations of regular women into brides.

Still, what lulls us into the comfort of a good-natured, finely crafted party-play has a serious core of loneliness, which is lovely. In the very center of that core, however, are characters with no life who spend far too much of it dreaming about and planning their weddings. I question whether this is the way things are right now. If so, if that’s all there is, this could be the saddest comedy I’ve ever seen.