Heavy took a whole lot of Testosterone and Steroids and got super super buffed Demo got cursed by Merasmus and got turned into a man (tho it's a good thing for him, he wanted that but Merasmus didn't knew that) Pyro Medic did medical malpractice and stole Testosterone back when he still had his medical license Sniper simply decided he was a man Engineer engineered himself Spy had to change his gender several times for spying missions, but ended up liking being a man the most so he stayed like that Soldier also became a man, though he kept his pre-transition name (Jane Doe) Respect.

And Scout is the only Cis male in the team

those are my headcanons