It may be one of the last conventions left in an increasingly unconventional culture.

But even as marriage rates decline out in the real world, TV seems fascinated with an odd offshoot: the Mormon practice of polygamy.

On HBO, Sunday marks the final episode of its polygamy-centered drama, “Big Love,” wrapping up a universe of convoluted plots and conflicts after five years. Besides handing meaty roles to “Titanic” alum Bill Paxton, film legend Harry Dean Stanton and talented eccentric Chloe Sevigny, we have this series to thank for introducing the world to star-in-the-making Amanda Seyfried and “the Principle” — the term patriarch Bill Henrickson uses for their, um, unique marriage setup.

And as the HBO series winds down, TLC revs up the second season of its unscripted look at a real-life polygamous family, “Sister Wives,” airing in the same 9 p.m. time slot as “Big Love.”

Each show tackles its subject in subversive ways. “Big Love,” which its creators always posed as a nonjudgmental take on polygamy, has instead become a treatise on the hypocrisy of its characters, who fall short of their religious and moral ideals with drama-filled regularity.

Most recently, Paxton’s Henrickson, elected to the Utah State Senate as an open polygamist, discovered his third wife lied about her age. She was 16 when they wed, and now his political enemies are using statutory rape charges to end his public stand for multiple marriage.

Stack that story line on top of Henrickson’s conflicts with his murderous, closeted gay brother-in-law who runs a cultish compound like a new-school Warren Jeffs, and you can see why real-life polygamists haven’t exactly embraced the series.

If “Big Love” shows its characters falling short, “Sister Wives” is a too-brazen attempt to slap a Brady Bunch-style sheen on a practice long considered illegal in most states.

On Sunday, patriarch Kody Brown, his four wives and 16 children prepared for the kids’ entry into public school while simultaneously readying for an appearance on the “Today” show in New York.

Through a series of candy-coated scenes, producers work hard to show the Browns are Just Like Us, with talk of domestic duties and school matters skirting the elephant in the room. (If Kody really believes “love should be multiplied, not divided,” as he says in the show’s opening credits, why is he the only guy in this arrangement?)

Such valentines to polygamy — kids joke about being “poligs” — gloss over the issues “Big Love” tackles head on. How does a modern woman justify living in a household where four women are aligned with a single man? How are friendships and work lives affected by their fame and resulting police investigation? (Kody Brown is legally married to only one of the four, avoiding obvious bigamy charges.)

“Sister Wives” this season also faces a common problem with notable unreality shows — the second season focus on the stars’ growing fame. So we see a family which has lived before TV cameras for months get tongue-tied visiting the Today show, fretting over harsh opinions on the Internet.

It’s odd to find a fictional show cutting closer to the bone than a series about real people. But while “Sister Wives” works hard to convince us of its stars’ normalcy, “Big Love” probes the difficult emotional truths of characters whose belief in their own morality is constantly challenged.

‘Big Love’ series finale

When: 9 p.m. Sunday

Where: HBO