In SM3, she now knows Peter's situation. Ok cool, but then, feeling ignored and jealous, she starts seeing Harry on the side the same way she saw Peter on the side when she was girlfriend to Harry in SM1 and JJJ's son in SM2. And not just once, but throughout the entire movie, and Harry is not just some guy either, it's Peter's best friend ever, and he's also her ex from high school that wasn't that long ago at the time. And Harry makes her break up with him! I know MJ is not genius level or anything, but no offense, if Spider-Man is your boyfriend, you can tell him what's wrong and work together on how to fix it, not all of a sudden just do what your ex told you to do. And for her to get mad that Peter went on a date after she broke up with him, after all the build up of the first two movies, it's just you can't sympathize for that character when she knowingly caused all of it. Then all Spidey saves her once more and their closest friend, that's so at the heart of their trio dies, they just brush it off literally after his funeral and have their happy ending. I will say, the upside down Spider-Man kiss was an ******* move on Peter's part, but MJ did the same thing as well in SM2, so that's karma. However, that's still not a good reason for MJ to leave the restaurant simply because Gwen happened to be there as well.