Stacy Keibler Marries on the Rebound -- George Clooney ... Doesn't

Stacy Keibler broke a basic rule of romance -- never marry the rebound guy.

Stacy -- who famously dated and broke up with George Clooney -- and new hubby Jared Pobre shocked their family and friends Saturday by getting married on a beach in Mexico during a surprise ceremony ... and -- get this -- they told People they'd been planning it for months.

Weird ... 'cause back in July '13 she told GMA marriage was the "last thing on her mind."