My point was that I've seen trailers for 2D movies attached to 3D movies, and I've seen trailers for 3D film attached to 2D movies (obviously the trailer was in 2D, also).
Also, A 3D movie isn't ALWAYS shown in 3D everywhere. My 14-screen theater often (but not always) has a 3D film on two screens -- They have a showing of it in the 3D IMAX screen, but also have a showing of it on a 2D screen. Other times, a 3D movie may only be show on the 3D screen for the first 2 weeks or so, and then it is only shown on the 2D screen.
I often go to the 2D screening of a 3D film, rather than the 3D screening.