Credit: Splash News Online; FameFlynet

To paraphrase Chandler Bing: Could the future Mr. and Mrs. Theroux be any hotter?! Jennifer Aniston and fiance Justin Theroux arrived late Monday, Aug. 19 in Cabo San Lucas, Mexico for a vacation, with pals Jason Bateman and his wife, Amanda Anka, also in tow. After checking into their resort, Aniston, 44, and Theroux, 42, wasted no time in doffing their clothes and hitting the pool -- with photographers capturing the stunning results.

Wearing a pink bikini she's worn on past vacations, the We're the Millers actress looked sexier and more in shape than ever -- with a completely flat, taut stomach, to boot -- as she lazed by the pool. Her actor-director fiance, meanwhile, didn't look too shabby either, revealing seriously cut abs and pumped-up pecs as he hung out shirtless in the Mexican sunshine.

A source tells Us Weekly that Aniston has been following "her normal Cabo routine," which includes not just R and R but fitness, naturally: "A yoga/pilates workout in the morning, and a long afternoon of tanning by the pool. Justin joined her for the workout and also by the pool."

Engaged for just over a year, "Jen and Justin seemed inseparable and totally in love," the source adds. "They swam in the pool together and at one point stopped and kissed. She swam up to him and put her hand on his arm and started kissing him. They were very passionate."

Bateman and Anka stayed by the couple's side as they lounged by the pool in the afternoon, with Theroux listening to his iPod as Aniston gabbed with the Batemans -- and sipped smartwater. (Aniston is a longtime spokeswoman for the brand.)

Adds the insider: "It is her favorite vacation spot!"