“Zack and Miri Make a Porno” is crossed with “I Love You, Man” in Lynn Shelton’s ostentatiously indie (read: scruffy) comedy-drama about two old pals, one a happily married guy and the other a would-be bohemian, who agree on a dare to make a flick showing two straight guys having sex and then try to go through with it. The title “Humpday” comes from the Seattle erotica festival that’s meant to be the place where their effort will be entered in competition.

This little movie comes from the bare-bones movement that’s been christened, rightly or wrongly, as mumblecore, and as Ben, the wedded half of the duo, it stars Mark Duplass, who, in tandem with his brother Jay, has made several ultra-low-budget movies (the most recent being “Baghead”) that fall into that group. Duplass proves a genial fellow, and he’s well paired with shaggy-haired Joshua Leonard as long-absent friend Andrew, who shows up on Ben and Anna’s (Alycia Delmore) doorstep at 2am one morning for an unannounced visit. Before long Andrew’s lured Ben to a “Dionysian” party at the hippie-like home of lesbians Lily and Monica (Trina Willard and writer-director Shelton), where they hear about Humpday and drunkenly hatch the idea for their movie.

It’s not until much later, when Ben and Andrew are in a hotel room attempting to overcome their embarrassment and do the deed, that anybody raises the question of why the notion of two straight guys having sex would be an earth-shaking artistic statement. And that’s a problem with “Humpday”: the premise is a thin reed to hang a script on. In any event it’s only an excuse to raise “serious” issues about homosexual inclinations in even the most determinedly heterosexual people, the limits of friendship between guys, and the strains male bonding rituals put on married couples.

Some of the observations that the script (largely improvised by the cast) makes are good ones, nicely dramatized. The difficulty that Ben has revealing his plans to Anna and the stumbling way in which the amiable goofball Andrew does so, for example, are well staged, with especially good work from Delmore, even if in themselves the plot turns aren’t much different from what one would expect in a studio product. On the other hand, the dialogue sometimes sound stilted, as though the actors were reaching for a profundity that’s out of their grasp, and the final sequence between the guys in that hotel room does run on. And the film doesn’t so much end as just peter out, so to speak.

As with so many of these little indie films, “Humpday” has moments of insight and cleverness, but it’s pretty threadbare in every sense. Even in the low-budget world, bromance may have had its day, hump or no.