Not all romantic comedies should have oh-so-pretty people spouting off gushy, mushy monologues that are lame even by standards of the most so-cute-it’s-sick lovebirds you know.

Still, that extreme is far preferable to Failure to Launch’s unlikable lowlifes who are perfect for each other, considering that if they never met on earth, they might someday cross paths in hell.

OK, Matthew McConaughey’s bronzed, buffed irritant Trip is more of a charmless, womanizing jerk. He’s a commit-o-phobe boat broker without a boat — as unlikely as a car salesman riding a Schwinn to work — who, at age 35, still lives at home with his parents. Trip also refuses to come clean with someone in his life about the truth behind their relationship.

But Sarah Jessica Parker’s “professional interventionist”? She’s gonna burn.

Her Paula’s job is faux-seducing aging guys who’ve never moved away from home as a lure into the larger world. (Trip is her latest project.) Once they’re out of there, she dumps them and collects her fee from the parents. And hey, what screams self-confidence and independence to a basement dweller more than the swift rejection of a beautiful woman? (Oh, and her no-sex policy goes out the window at a crucial moment in Trip’s case, so toss glossed-up prostitution in there, too.)

Paula makes Hitch look like a patron saint, and the movie she’s in is Hitch with no inspiration, soul or swoon to be found. (Parker got more laughs and honesty from a similarly themed episode of Sex & the City that dealt with a hunky comic-book store owner.)

With its ludicrous plot points, Failure to Launch drives a stake into almost anything resembling a heart. The sole exception is the one scene Kathy Bates, playing Trip’s mom, must have signed the dotted line for — a bit near film’s end about her true, deep-seated fear over Trip moving out that’s worth the one star.

Everything else spells out why this film abruptly was pushed back from Valentine’s Day weekend to the middle of March. It’s filled with any number of bizarre, awful bits.

A scene of mountain-biking action resembles a commercial for a prescription drug Trip might use to combat what he’d pick up from his conquests. Paula’s roommate (Zooey Deschanel, unable to turn tin into gold) is a lifetime drunk who wants to kill the loud mockingbird outside her window. A paintball visit (suggested as therapy for what Trip thinks is a recently deceased pet) is filmed like a cop shootout, complete with the movie sound effect of emptying assault-rifle clips.

It gets worse. Trip steals his clients’ boats for joysails. A sage, precocious kid with no front teeth utters the words “lesbian” and “bastard” for laughs. We’re offered no fewer than three shots of Terry Bradshaw’s (as Trip’s dad) sagging butt and gut while he lounges naked listening to Dr. Dre.

A chipmunk, dolphin and chuckwalla (think iguana) attack Trip because, per a friend, his life is out of balance with nature, so it’s striking back at him. After inflicting what could be a broken-back injury, the chuckwalla chuckles at Trip. At least this stuff is entertaining to one breathing creature.

All of this leads up to a protracted, laborious setup where Trip and Paula, after all the lies and confessionals play out, attempt a reconciliation that ends up being broadcast on TV at a café.

Along with a significant rewrite, or 12, the movie might be better with Deschanel and Patton Oswalt — the King of Queens co-star has a small role as another target of Paula’s — in the leads.

Oswalt at least resembles someone who might have, for whatever reason, never left the nest, and, at some point, Deschanel and her zingers must rise up from second-banana roles in third-rate material such as this decrepit, classless comedy.