"Who throws away their marriage to save their old half dead Aunt?"
I don't know? A superhero maybe? How about two young people who can't live with the fact that their marriage would be based on their decision to let someone else die.
I hate OMD because of what it did and how it did it. It made MJ and Peter both look bad by design. Peter can't let May die because of him. He especially can't let May die if there's an option to let her live. MJ gets royally screwed in this. She knows that on one level, Peter would resent her for making him pick her over May and she can't live in a relationship like that. She also knows that, by design, there's only one real option here. If they take the deal, no one dies. And please stop with the daughter. Read OMD again. Mephisto doesn't release the daughter thing till after the deal is already made. And even then she's just a possibility... So, if they take the deal they lose their marriage but no one dies. This is the herioc option. If they don't take the deal, someone dies and they live with a selfish decision.
And as Tendrin pointed out, the reasons some of you are making are just sophmoric and juvenile. "She's old." Wow. Breath taking observation. "She doesn't do anything." Do you get Spider-Man comics? "Who would trade their hot wife for their old Aunt" ... ... Spider-Man would. He so would. He'd do anything for Aunt May.