Neither Mary Jane nor Peter
had sex with Mephisto 
- not sure where you got that from. Although Mary Jane was the one who accepted Mephisto's offer to sacrifice their marriage in return for saving Aunt May's life, it was stated quite clearly that this would only work if Peter agreed to the deal as well (an important distinction to those who say that Peter is blameless in the whole thing since it was Mary Jane who sealed the deal, etc).
The story behind the identity-reveal erasure hasn't been told yet, though the issue has been dealt with a little bit in the team-up with the Fantastic Four a few months back.