The old box of popcorn trick....I fall for it every time!! lol
Even though a movie theatre is dark doesn't make it private.
Maybe she has a little more respect for herself than you have for her?
Not every woman wants groped in a public place where there's a possibility of someone they know noticing what's going on.
Get a room, get a car, go to a drive in instead of a walk in theatre.
If all else fails, buy her an engagement ring.