This only applies when it's possible. If the toilet is full of people, you take any free spot.
The reason for leaving empty stall between in an empty toilet is to give space and let people piss in peace. Same way you don't sit right next to a random person in a park if there's other benches free.
at that point, who gives a shit? i don't assume the relation status of people sitting in the theater. i was once at a movie with a colleague of the opposite gender, just because we both wanted to watch it.