I had a girlfriend once and we did something very bad on a beach in Kauai. There were no kids around, closest people were about 30 yards away and she sat on my lap - kissing me (but we were doing A LOT more).
Honestly - as long as there are no kids around, and you aren't interrupting the movie and you're not overtly trying to get anyone's attention ... I really doubt anyone will say a damn word as long as the theater isn't crowded. Pay in cash for the tickets so there is no credit card receipt hanging out there to identify you if you have to suddenly haul ass out of there.
I mean seriously - you should be able to work with this ...