There isn’t a false moment anywhere in “Humpday,” not in the writing, the directing, the performances, or the dynamics of the characters’ relationships. What’s especially impressive about this is that the story presents a few unlikely scenarios that nonetheless come across as believably as everything else. You buy every minute of it, even the part where two completely heterosexual best friends decide to make a sex tape.

Behind that provocative premise is a comedy that’s consistently funny, as well as genuinely insightful about the way men relate to one another. It may sound like nothing more than the gay version of “Zack and Miri Make a Porno,” but for one thing, it’s not gay. It’s something else altogether. “Zack and Miri” started with its salacious idea and worked backward from there to come up with a story to surround it; “Humpday” feels like it started with questions about the nature of masculine friendships and arrived at the friends-make-sex-tape element as the result of those questions.

The friends are Ben (Mark Duplass) and Andrew (Joshua Leonard). Ben is recently and quite happily married to Anna (Alycia Delmore), has a grown-up job, and is a productive member of society. He and Anna are talking about having kids. They have a cute house in Seattle. Andrew, Ben’s old college friend, is one of those freewheeling artsy types who have no regular source of income and live from one bacchanal to the next, the type of guy who might, as Andrew does, show up at your house at 2 a.m. unannounced. “That’s classic Andrew” is Ben’s best explanation.

Also an example of classic Andrew is meeting a girl at a coffee shop in the afternoon and being a central figure at a house party she’s hosting that same night. The Pacific Northwest is full of people like this: polyamorous, anything-goes hipsters who celebrate Art and Truth and marijuana (at the very least). Andrew is at home among them. Ben stops by for a bit, embarrassed to be wearing a necktie (he came straight from work), intoxicated by the brief blast of hedonism that he’s not at liberty to enjoy much anymore. The house has a sign on the front door that says “Dionysus” — “and they’re not kidding,” Ben tells Anna when he calls home to check in.

It’s during this party, after a lot of alcohol and weed have been ingested, that the topic of Seattle’s do-it-yourself-porn film festival arises. Ben and Andrew decide — and it’s quite something how organically the conversation arrives at this, like it happened accidentally over the course of two actors improvising a scene — that the ultimate artistic statement would be if they, two straight but loving friends, had sex on camera.

It’s a joke at first (Ben calls information to get the number for the “Bonin’ Hotel”), and they feel differently once they sober up. But the topic has made Ben start thinking about deeper issues, like wanting to prove he’s not as hopelessly “white picket fence” (i.e., dull) as Andrew thinks he’s is. For his part, Andrew wants to actually finish an art project, which he’s never done before. It’s not about the sex — there is nothing erotic or romantic about it for either man — but it’s about Something Else.

The film, which was written and directed by Lynn Shelton (who also plays one of the women at the house party), has the aesthetics of a Mumblecore film — ad-libby dialogue, hand-held cameras, Mark Duplass, etc. — but with more discipline and less meandering than is usually evident in those films. It also has farcical situations — wrong impressions, double entendres, and so forth — but portrays them realistically, without exaggeration. The scenes of Ben trying to tell Anna about the project, and Anna and Andrew discussing it over a bottle of Scotch, are priceless for that very reason: On paper, these circumstances sound borrowed from an episode of “Three’s Company,” yet the film never goes over-the-top or silly.

Male viewers are bound to recognize themselves here and there. If it’s not in the way Ben and Andrew’s project becomes a matter of pride (both want to chicken out, but neither wants to be the one to veto it), then maybe it will be in the way they can move from deep emotions to wise-cracking and back again in a 10-second span of conversation.

Whether or not they actually go through with it is almost beside the point, ultimately, because it was never about the sex anyway. It’s about friendship, male bonding, platonic love, and society’s concept of masculinity. I think one line from Ben sums it up perfectly: “I think we might be morons.” Yes, probably. Human, believable, lovable morons.

A- (1 hr., 32 min.; R, a lot of harsh profanity, brief strong sexuality, some nudity.)