Re: X-Men First Class v.s Captain America TFA
The difference is the story in First Class was better served by it happening. The Magneto/Xavier breakup is far more effective if Magneto's actions results in Charles being broken. Having it occur at a later time would weaken the impact of the event. It belonged in First Class, which was ultimately a story of the origin of the X-Men and Brotherhood.
Captain America on the other hand is essentially two stories. One about the origin of Captain America and then a second about him being frozen and waking up in the present. The latter is just tacked onto the end of the first and it doesn't mesh well, with the latter being tremendously shortchanged.
Back to Mystique, Magneto is offering her what she wanted from her very first scene in the film (as an adult). He is the only one in the entire film who accepts her for what she is (aside from maybe the villain Shaw, whom she had little interaction with). She was in a position where she had to make a choice, and she was still going to stay with Charles for his sake even though her heart wanted otherwise. Just because you wouldn't have listened to Charles in the same situation, doesn't mean other people wouldn't. Given what we learned about her character in the preceding two hours, that was the decision that the character should make. Her abandoning Erik for Charles is what would have been out of character.