Math forced Shonda Rhimes to do it. Having exhausted every romantic variable for her

Grey's Anatomy

surgical team in less than three seasons, the creator and executive producer was forced to establish a new franchise in California in order to start romping anew.

So Thursday night, in a two-hour extravaganza, Grey's Anatomy - a massive helping of sex with a modest side of surgery - begat the Unnamed Addison Montgomery Spin-off starring the fetching Kate Walsh, all legs, attitude and gymnastic eyebrows. Jugglers have done less with three pins and a bowling ball.

The episode, dubbed "The Other Side of This Life," cut between gray, hermetic Seattle Grace Hospital and sunny Southern California Oceanside Wellness Clinic, where Addison travels to visit old med-school pals, make out with Tim Daly, and talk to an elevator.

Naturally, the self-professed "adulterous bitch who forgot to have kids" has found her new home.

Once titled Private Practice, the potential spin-off of ABC's top hit - the network will announce the fall lineup later this month - costars Daly, health guru Taye Diggs, his ex-wife, fertility specialist Merrin Dungey (divorced for some tenuous reason still eluding logic), weepy shrink Amy Brenneman, neurotic gynecologist Paul Adelstein, and surfer/receptionist dude Chris Lowell, who will be contractually obligated to stride shirtless in slo-mo on every episode.

Rhimes and co-executive producer Betsey Beers know how to get busy. Within the first two hours, every cast member has been potentially coupled off.

Thursday's episode featured a slatternly sex surrogate, a sexually frustrated wife exercising herself into ACL surgery, an Internet date with "sexyboobs316" resulting in the destruction of a Porsche 911 (cars being an alternative form of California porn), ogling of surfer/receptionist by women doctors, canoodling, near-canoodling, talking endlessly about canoodling, and, oh yes, a modicum of medicine.

And drinking, the basic difference between dramas and sitcoms being that characters imbibe on the former and eat on the latter.

Grey's Anatomy, run by television's first African American female show-runner, features a racially and ethnically mixed cast of characters who constantly inter-date as long as they're both surgeons. The spin-off cast is, so far, less diverse, though it does introduce - wait for it - Jewish doctors. There are no gay characters on either show. Then again, there are no nurses, lawyers or ugly people. But the absence of lab coats and scrubs on the L.A. incarnation allows for decidedly more leg and pecs.

Sex on the Beach (or whatever this ends up being called) has no character as indelible as Grey's irascible Miranda Bailey (Chandra Wilson). But Walsh and her seasoned castmates seem adept at playing comedy and pathos, with an accent on the former. Walsh is that rare actress who can be moving yet funny with a dozen acupunture needles piercing her face.

Addison Montgomery-Shepherd (sounds more like a brokerage firm than a neonatal surgeon), Dr. Derek's estranged wife who slept with his best friend, was introduced in the finale of Grey's debut season. Originally, Walsh was signed to five episodes in order to wreak havoc on the Luke and Laura of Seattle Grace, anhedonic Meredith Grey (Ellen Pompeo) and Dr. Mc-Let's-Not-Go-There, the only surgeon ever to live in a trailer (perpetually pink-nosed Patrick Dempsey).

But the producers forgot the Locklear Law of Broadcast Dynamics: Every drama demands a shrew in short skirts.

Mercifully, so far in Addison's move to Southern California, there has been no introduction of a Mc-Something character. Let us hope that Rhimes and company keep it that way.

Contact staff writer Karen Heller at 215-854-2586 or kheller@phillynews.com.