My transfemme Peter Parker take is that, given how often MJ gave Peter fashion advice in the original comics (and her genuine love for fashion in general), she would be so fucking stoked to be able to go clothes shopping with Parker once she comes out. Like as soon as Parker comes out to MJ she'd be going through her own closet to find clothes for her to last until they can make time to go on a big shopping spree, it would be one of the highlights of their relationship in MJ's mind.
Ok my shower thoughts decided this needed an addendum, so here we are.
I actually think transfemme "Peter" Parker makes a lot of thematic sense as plot development for Spider-Man. The crux of Peter's whole character in the comic is this pull between what he feels he needs to be vs. what he feels others want him to be. The people in Peter's life want him to be reliable, social, approachable, and "normal" for lack of a better word - to lock in, get a steady job, and make a financial and social success of himself. But while Peter wants to meet those expectations because he cares about those people and their opinion, he feels he NEEDS to be a hero who uses his powers to fight injustice, even when it brings him nothing but trouble and makes meeting those expectations put upon him impossible. He can't even reveal to his loved ones that he's the one doing all these heroics - and thus explain WHY he keeps failing to meet their expectations - because he's afraid that if everyone knew who he really was, not only would he be in danger, but so would the people who love him.
And of the two personas - Peter Parker, who tries and fails to be the person others want him to be, and Spider-Man, the person he feels he MUST be no matter what - Peter vastly prefers Spider-Man. Modern day comics may make that seem like a crazy statement, but, like, take one of the less-remembered lyrics of his 60's theme song into account: "Wealth and Fame, he's ignored, action is his own reward." The action of being a hero is what he lives for. He cracks jokes, he revels in his physicality, he's so much more jovial and lively, hell, his clothes are bold and colorful compared to the drab colors he tends to wear as Peter. Spider-Man isn't just who he feels he needs to be, but who he WANTS to be, even if it brings him hell from time to time.
We have this character who is constantly crumbling under the pressure of what people expect him to be - and not just evil people, but even well-meaning people who care about him - and who has a secret identity where he gets to actually be his true self. And it's the secret self he prefers, the one he overwhelmingly chooses to be at the expense of his public self, even though doing so causes him guilt and strain in his personal relationships.
Yeah, that's a trans experience I think!
I also feel like this is why she definitely comes out in costume first - Spider-Man becomes Spider-Lass the second she finally makes the choice to homebrew her own HRT (it can't be harder than figuring out web-fluid was, after all - or, like, the countless cures for the Lizard's whole schtick she's had to figure out in the past), whereas "Peter" tries to remain in boymode until it's just physically impossible to hide that some changes have happened. And, as is often shown to be the case in Spider-Man comics, most of the people in "Peter's" life are way more accepting of the truth than she thought they'd be - although, again as the comics also show, there are still some people who choose to be assholes and find a way to make her pay for her moment of honesty.
Norman Osborn would be absolutely horrendous about it, but that's ok, it just makes him all the more satisfying to punch.
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AND MARY JANE WOULD FIGURE IT OUT BEFORE "PETER" REVEALS IT! That's important! Even if this takes place before "Peter" finds out she's known she's Spider-Man for almost his entire career, even if it's before they hook up, she would figure it out at least as soon as Spider-Lass comes out of the closet, if not before (and she would at least have some suspicions before "Peter" comes out), because Mary Jane is great at reading people. And, in her fashion, she would be preparing some resources to help "Peter" when "Peter" finally opens up, and maybe even slyly find a way to get her to stumble upon them on her own - because that's exactly what MJ did to help "Peter" with superheroics in the comics. MJ would be supportive as hell and it would be very sweet!
