Drew Wall, Natalie Young and Alex Organ in Andrew Rapp’s Red Light Winter from Second Thought Theatre

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As staged by Second Thought Theatre, the extended sex scene in Andrew Rapp’s play Red Light Winter is the most realistic, most compelling scene of its kind on an area stage in years, probably since Franz Xavier Kroetz’ Through the Leaves at the Dallas Theater Center in … oh, 1987.

When flesh gets exposed onstage — especially if there’s simulated sex involved — we audience members tend to get very quiet. We feel self-conscious (or try to show how un-self-conscious we are). It’s one reason that despite the widespread notion that nudity makes for hotcha! box office, it actually doesn’t. Not in live theater, at any rate. Sexy sells, not overt sex.

Audience studies have proven this, but case in point: The sex scene at the end of the first act left me strolling out at intermission, still marveling at it. A young couple seated nearby walked out — and never came back. Yet we’d all been warned beforehand that Red Light Winter contains full-frontal nudity and “strong sexual content.”

So much for being a safe bet with ticket sales. In the theater, audiences bear witness to the actions and speeches of living people in the same room as us, breathing the same air. That’s the great, humanizing power of the stage and its great source of embarrassment. Nothing makes you feel quite so awkward as watching a person make a naked fool of himself in front of you. We’re trapped with these folks; they exasperate or they exalt.

What also makes the Second Stage’s sex scene a technical marvel is that it takes place in the Addison Centre’s Studio Space, a small, black-box rehearsal room. We’re practically in the actors’ laps. Director Regan Adair had to choreograph the scene meticulously with his performers, Drew Wall and Natalie Young. But of course, it can’t seem like it’s planned. It has to be convincing and natural, consistent with the characters and, yes, tender, even erotic.

They succeed, marvelously well.

The sex scene merits such detailed discussion not just because it’s sexy and I’m dirty-minded. It’s actually the pivot for Rapp’s entire play. Everything in the first act of Red Light Winter leads up to it, and everything in the second spins away from it like a car on ice. If the scene doesn’t work, the play sinks. Here, it keeps a long and leaky play afloat.

We’re in Amsterdam with two closing-in-on-30 New York males, friends since college. Matt (Drew Wall) is a struggling playwright and obsessive-compulsive whiz, still recovering from a wretched intestinal ailment, still un-recovered from a girlfriend.

Which is why his buddy, Davis, has brought him here. A brash, hard-charging lout, Davis (Alex Organ) is a hotshot book editor. (That’ll date this play fast — can anyone be a hotshot book editor anymore?) Despite his boyish obnoxiousness, Davis does mean well, more or less: He hopes to jolt Matt out of his misery by hiring a Dutch prostitute for him.

Christina (Natalie Young) is French, not Dutch, and she’s clearly smitten with Davis, the Big Ugly American, and not Matt, the Small Nerdy American. But she was hired to do a job. What she wasn’t hired to do was entrance both men by singing a song — and leaving Matt with a life-changing memory of love found and lost.

Which, by the way, is the other reason we hold our breath during that sex scene: There’s such brokenhearted yearning, we fear the tenderness is bound to go horribly wrong somehow.

In tone and temperament, Red Light Winter owes too much to the street-weary romanticism of early Tom Waits (whose songs are prominently featured). We have a sweet, young hooker here — a chanteuse no less — who falls for one of her customers. What low-grade 1940s film noir did she stroll out of? I love Waits’ work but he was smart to chuck the boozy, Charles Bukowski-poet-of-the-losers pose and aim, instead, for an art that’s more oblique, more tough-minded, less self-pitying.

And self-pity is an issue here. Red Light Winter is often mordantly funny — like a Judd Apatow buddy flick, complete with dope jokes and slovenly male housekeeping. But it’s a buddy flick that wants to be a brutal Neil LaBute drama. It’s even got the masculine sexual competitiveness, the cad vs. the wimp.

This is where the self-pity comes in. In the first 20 minutes or so, we’re pretty certain that the pathetic Matt — the stand-in for playwright Rapp (above) — is going to get crushed somehow. And Davis looks like a good candidate for the crushing: He’s less an adult human and more a dramatic contrivance (the revenge of the poor playwright on the jocks who bullied him in high school). Davis is an aggressive, frat boy past his maturity date. But he also knows it — and still cheats on his fiance, still pranks poor Matt. He remains cruel yet has enough self-consciousness to recognize how cruel he is. This is like Donald Trump with doubts; he ceases to be Trump. The only thing interesting about Davis is the edge of bitter rage that Organ gives him. It makes us wonder what did Matt (or somebody) ever do to prompt such inner conflict?

All of which is to say, as director, Regan Adair has skillfully redeemed Red Light Winter, made it feel real, made it such an engaging entertainment, even as it’s too long for its own good. (His success makes one feel, again: Too bad Adair just moved to New York). All three of his actors are fearless: Organ for being willing to make Davis unpleasant, not just a manly carouser, and to do it with unrelenting commitment; Young for her emotional vulnerability, the way she makes Christina seem quintessentially French: forlorn and knowing, voluptuous and physically still, more worldly and seemingly older than the American males who practically jump around like dogs, eager to please. Yet in the second act, Christina is truly naked: All of that pose is gone, like makeup rudely scrubbed from her face.

As for Wall, I’m surprised films or television haven’t snatched him up yet. He’s a classic comic-sidekick character actor, the rabbit or the weasel, a Charles Martin Smith or a Steve Buscemi in their younger days. He was perfect as the mumbling psychic henchman in the Undermain Theatre’s The Dog Problem, but Red Light Winter allows him to extend that type, give him pathos and detail.

Despite its artsy pretense of squalor and seriousness, Rapp’s play is not that realistic (the big plot twist feels calculated), and Matt, despite his yearnings and anguish, is not fully tragic. Mostly, he’s life’s punching bag — when he’s not delivering the play’s deadpan jokes.

But punching-bag or punchline, Wall wins with both.