If Peter was trans, May Parker would support him. Tony Stark would not.
I'm disgusted by prop-Tony fics that have May throwing Peter out because May is homophobic or transphobic. The fics trash a wonderful character as a plot device to get Peter in Tony's penthouse (and sometimes Tony's bed, ew). This scenario is the opposite of canon: May loves and supports Peter, while Tony Stark blackmails Peter, then ignores him. Tony Stark is also both homophobic and transphobic in canon, based on his "humor" about gay and trans people in Iron Man 1.
So how would the blackmail scene in Peter's bedroom go if Peter was trans?
***
Peter frowned as Mr. Stark continued to ignore Peter refusing to go to Germany. Then Mr. Stark threatened to tell Aunt May. Peter shot the web sticking Mr. Stark's hand to the doorknob as he considered.
May--and Uncle Ben before he died--had already done so much for Peter. They'd treated Peter as PETER since the day he told them he didn't want to wear dresses anymore because he was a boy. They'd done everything to support his decision, even helping him to legally change his name and seal his records. May and Peter had talked about what would happen when his body started changing. Peter was grateful that so far, not even the spider bite had made him have a period.
How could Peter dump Spider-Man on Aunt May? He couldn't. So instead he lowered his head and said to Mr. Stark, "I'll go to Germany."
After he'd freed Mr. Stark of the web fluid, Mr. Stark said, "Great! We'll tell Aunt Hottie you're going to visit the Tower for some internship stuff. And I'll get your suit whipped up--I've already made some designs and modifications."
Mr. Stark pointed a device at Peter and pushed a button before Peter could refuse. He heard the device beep and watched Mr. Stark's eyes open wide.
"WHOA, Peter Piper doesn't have a pecker," Mr. Stark said as he stared at the scans. Then he looked at Peter and said, "I've joked about unmanned missiles, but this is a first. I thought the birth certificate was a mix-up. You really ARE Melissa, aren't you?"
"My name is Peter," Peter said.
Mr. Stark shrugged and said, "Sure, Missy, don't worry your pretty little head about it. So, let's talk about Germany. It's going to take an extra hour to alter your suit to cut out the space for your...junk."
Mr. Stark grinned as they turned to leave and said, "Wouldn't J. Jonah Jameson love this headline: Spider-Man or Spider-Girl? Only the suit knows for sure."
Peter stopped cold as Mr. Stark joked AGAIN about something so very private. Peter remembered how Ben and May had taken the transition to Peter so calmly. They'd never done anything except love Peter and fulfill his trust in them.
Peter straightened as he followed Mr. Stark into the living room and heard him lying to Aunt May about what Peter and Mr. Stark would be doing.
Peter interrupted by saying, "May, he's lying to you about everything."
As both Mr. Stark and May turned toward him, Peter stepped forward and said, "I'm sorry, May. You've been so great about--about PETER. I should have trusted you with this as well: I'm Spider-Man."
Before May could react, Peter continued, "Mr. Stark threatened to tell you I was Spider-Man if I didn't go with him to Germany for some secret mission. And I was gonna go, but then Mr. Stark made a joke about exposing me to The Bugle. And I realized that I couldn't trust HIM with anything, but I can trust YOU with everything. Even Spider-Man."
May smiled at Peter and opened her arms. He flung himself into the hug and blinked back tears.
He could imagine May's glare as May said, "Mr. Stark, I don't know anything about how rich people operate. But I KNOW blackmail is wrong. And blackmailing a CHILD...Get out. Get out and NEVER come near Peter again."
Peter lifted his head to see Mr. Stark open his mouth to protest, but May's fierce look shut Mr. Stark right up.
As Mr. Stark headed to the door, May said, "I promise you this, Stark. If ANYTHING ever comes out about Peter--who he is or who he chooses to be--I will END you. And not even the Iron Man armor will stop me."
As the door shut behind Mr. Stark, Peter hugged May tighter as she said, "Peter, I will ALWAYS love and support you. As Melissa, as Peter, and as Spider-Man. Always."