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You shouldn't pretend you like something that you don't - unless you tell him the truth, he's gonna expect you to want to see all the superhero movies with him. If you guys were already dating, I'd say you should suck it up and watch something he likes that you don't once in a while, as I'm sure he does it for you sometimes.. but since this is a first date it should be something you'll both enjoy.
Is there another movie that is out that you would prefer to see? If so, suggest it to him and apologize for saying you were joking about not liking Avengers and admit that you actually don't want to see it.