Ashton Kutcher & Mila Kunis -- Definitely Maybe Banging Each Other

Mila Kunis insists they're just friends ... but last night in L.A., the actress hit up a romantic Italian restaurant with her former "That 70s Show" co-star Ashton Kutcher -- and they even left in the same car.

The two have been rumored to be secretly dating for months -- but Mila adamantly denies they're anything more than platonic pals ... who eat out together alone ... and go on romantic weekend trips to quaint little towns.

It's extremely suspicious ... so we gotta ask ...