Why did Mephisto want to erase the marriage? What if he hid the truth in his lies? The truth being Peter losing his daughter with MJ permanently due to the bargain. We learn Norman had May but Mephisto realised she had great potential and wanted her but Norman was using his limited knowledge of magic to keep Mephisto or anyone from taking her. So he made a deal with Norman he'd get May and Norman would get his son back. Mephisto used Civil War and Aunt May's shooting to force Peter and MJ into the bargain by erasing the marriage he could pull May from continuity without any fears of Peter ever learning he had the child.