Also, it doesn't sound likely he would do any of those extra things you suggest without jacking up the price of the transaction.
Well, yes, that much is right there on the page. MJ asked Mephisto to restore Peter's life to the way it was and "give him a chance for happiness," and in exchange for that, she would "sweeten the deal" by offering him something. That's not in dispute. The question is simply what it was that she offered him, and I have never before heard anyone suggest that it would be anything as crass and simple as her having sex with the Devil. I mean, this is the Devil! If he wants carnal pleasures, he'd have no trouble getting hold of women who'd do far kinkier things than MJ has ever contemplated. I think he'd consider that far too trivial a reward to be worth his while. I mean, he's the Prince of Darkness, not a frat boy. The kind of price he'd demand would be something that caused someone pain on a spiritual level. That's what gets him off -- not bedding a supermodel.
-It raped the mythology
That's an ugly thing to say. Rape is a hideous, life-changing violation inflicted on a real human being, and it's immature, crass, and insensitive to use it as a metaphor for someone telling a story you didn't enjoy. Grow up and get a sense of proportion.
-It was ridiculously out of character
What character? Part of why I liked "Sins Past" is that it actually gave Gwen Stacy a personality. Let's face it, until that story, Gwen was pretty much defined solely by one thing: her death. Before that, she was just a fairly ordinary love-interest character. And even in the story of her own death, she was nothing but a passive victim, a pawn sacrificed in the battle between two men. "Sins Past" redefined the story of her death so that the reasons for her death were about her, not just about Peter. It made her the central character in the defining story of her life rather than just a pretty prop to throw off a bridge so a male hero would be all angsty. And it makes her a real, nuanced human being with understandable, forgiveable human failings, rather than a shallow, idealized fanboy fantasy.
-It makes all the times Peter spent mopping about her death over the past 35 years kind of fucking pointless, now doesn't it?
What a hideous thing to say! Just because she was human and imperfect, her death wasn't worth grieving? What a truly horrifying sentiment.
She cheated on her one true love with his best friends dad! How is that not the definition of "Whore"!?
Because the definition of "whore" is "a woman who offers sexual services in exchange for money." That's not Gwen, obviously. She was a young woman who made a mistake because an older, powerful man took advantage of her, and who deeply regretted it afterward. People make mistakes. Interesting fictional characters make mistakes. Look at Peter Parker. His whole life is defined by one huge mistake -- letting a burglar escape justice when he could've easily captured him. He made that one mistake, he paid a high price for it, and he spent the rest of his life trying to make amends for it. Gwen Stacy did essentially the same thing. So unless you also think Peter Parker is an unforgivably awful human being, then you're employing a grossly sexist double standard.