If *today* a state like Texas or New Hampshire wishes to break away with consent of its population, should the Federal governments or people in the other states be able to prevent such divorce? Or would they be traitors for daring to ask? I though individuals could leave marriages? Silly me.

Mutual dissolution would be legal. If *today* Texas successfully negotiated a withdrawal from the Union then it would be fine. If it demanded independence by having its "militia" assault Fort Cavazos, then it would be treason.