Cameron Diaz & Benji Madden -- Smells Like Marriage

Cameron Diaz and Benji Madden are having a big shindig tonight ... and from everything we're hearing it looks like they're getting hitched.

We're told the party is being thrown Monday night at Cameron's mansion in Bev Hills. Trucks have been arriving all morning ... from catering, to The Velvet Garden florists, Classic Party Rentals, and Beverage Warehouse.

Cameron has been wearing what looks like an engagement ring -- a big one -- since December.

They've only been dating since May. It's interesting ... Cameron has been very vocal about being anti-nuptial, saying, "I'm not looking for a husband or marriage," adding, "I don't know if anyone is really naturally monogamous. We all have the same instincts as animals."