HBO is justifiably proud of its original series, but the pay channel seldom touts the various staples found on sister channel Cinemax — half-hour shows like “The Erotic Traveler,” “Sin City Diaries,” “Hollywood Sexcapades,” and the semi-serialized college frolic “Coed Confidential.”

Cinemax has long been a favorite punchline in TV circles — the ignored stepchild that’s earned the nickname “Skinemax” for its reliance on latenight movies that teem with bare flesh, invariably contain “Seduction,” “Naked” or “Forbidden” in the title and a few years ago seemed to consistently star Playboy pin-up Shannon Tweed. Less widely publicized, though, is the fact that the network airs an assortment of erotic series, albeit with plots that usually provide the thinnest excuse for characters to get naked and hop into bed, or onto another flat or tilted surface.

So who, exactly, puts the sin in Cinemax, and why do they bother investing in series? It’s not like anybody watches something called “Sin City Diaries” or “The Best Sex Ever” for the plots or acting, after all, though who knows how desperate viewers will become before fallout from the writers strike is over.

Historically, most of these programs are anthologies, but that’s gradually begun to change. “Coed Confidential,” for example, features ongoing plotlines and a central cast — sort of like “Friends” if Ross, Rachel and Joey skipped the preliminaries and got right down to business in season one.

Jill Champtaloup, VP of Cinemax program planning and scheduling, admits to occasionally being shot a sideways glance when she wears a network-labeled cap to a Little League game. Still, HBO is unabashed about its latenight lineup, which has become even more valuable since the advent of On Demand technology.

Cinemax orders at least three new series each year, Champtaloup says, acquiring programs that are created with the channel as a primary window and also marketed to hotel pay-per-view and on DVD.

According to HBO exec VP of program planning Dave Baldwin, the series possess only marginal appeal overseas, where they’re considered too tame to compete alongside hard-core porn. The U.S.’ slightly puritanical streak relative to Europe partly explains the popularity of Cinemax fare, which subscribers can watch as part of a single channel fee that includes Hollywood movies, as opposed to anteing up for pay-per-view titles.

“There is nothing that shows up on your monthly bill that could, for many people, get you into trouble at home,” he says diplomatically.

Cinemax insists, however, that it’s stepping up its game compared with the Tweed years, which emphasized extremely cheap anthologies. Mindful of data that shows women comprise roughly a third of its latenight audience, Cinemax wants to improve the storytelling within these shows.

“Believe it or not, over the last couple of years, we really have tried to develop the storylines and spend time on the characters,” Champtaloup says. “It’s a way to get people more engaged.”

Storylines and character development to illuminate why people flop into bed with each other? Seriously, is nothing sacred anymore?

“Coed Confidential” is a case in point, and will likely be back for a second season. The channel will also introduce a new series later this year, “Zane’s Sex Chronicles,” based on the popular African-American short stories. As a sign of the better-known talent being enlisted, Suzanne de Passe — whose credits include the miniseries “Lonesome Dove” and “The Jacksons: An American Dream” — is onboard as a producer.

If you think of pay TV as a quilt, all this makes considerable business sense. HBO and Showtime want to inspire a wide assortment of constituencies to subscribe, which is why prestigious series, original movies, theatrical titles and boxing all have their place, as do excuses to show off nudity and sex, even if it’s rather tame by pay-per-view and download standards.

As Baldwin says, “Some people aren’t quite ready for full-blown, triple-X pornography.”

Besides, in “Coed Confidential’s” season finale, when Ophelia wins a research grant, “The gang quickly plans a big bash in her honor where a number of surprising twists ensue.”

OK, so maybe the twists won’t be all that surprising. At least they’re sure to give it the old college try.