At the risk of honestly depicting myself as naïve back in the day, I'd say it has to be the first movie I ever saw at a theater -
Ode to Billy Joe. I viewed it with my girlfriend (later my [first] wife) in the summer of '76 in Spartanburg, SC. The sex scenes in the barn stalls freaked me out because I hadn't a clue as to what the fuck they were doing, let alone as to why, and what the hell such scene had to do with the rest of the movie, of which I equally was having trouble comprehending. All I knew is that I somewhat enjoyed the song when it played on the radio. Upon watching the movie, I sincerely thought that all movies shown at theaters were of the same nature, nothing like the movies shown on TV, later realizing that many of those were once shown in movie houses when first released.
Ironically, in the late 80's when I used to host Writers Night in Nashville, TN, I had the pleasure of having Bobbie Gentry as my special quest, she, of course, being the renown singer of Ode to Billy Joe. A week (maybe it was two) earlier, I had Jeannie C. Riley (Harper Valley PTA) as my special guest. Back to Bobbie, I relaid to her the story above, whereupon she comforted me (no longer needing any comfort since I was a dog) in saying that we all were naïve when younger (paraphrased).