Maybe she keeps telling that story because it's become part of your shared history now. Itās less about the ruined plot and more a running joke she connects with you. That kind of thing can turn into a weird badge of honor over time.
Being "that guy" is a life sentence, man. I once spoiled the twist in a different superhero movie by talking too loud in the lobby and I still feel the shame. Ten years from now, your wife will probably just say "the Batman thing" and you'll both know exactly what she means. We just have to live with our curse.