Something Zack had always found weird was how rigid a lot of people's ideas about sexes and gender were. As a kid, he'd been similarly close to boys and girls, and had felt like it was annoying that boys treated girls as gross and girls treated boys like him as gross. When he'd hit puberty, he'd fairly quickly realised he was bisexual, and so felt quite isolated for finding guys attractive as well as girls.
Probably the biggest thing to make him find the way guys and girls clashed with each other dumb was when in his biology classes they explained that everyone's body starts out female in the womb, and then if they receive certain hormones their body changes to be male. Finding that out fascinated him, because he always wondered from then on what people would've been like if they hadn't or had received those hormones. It also made him sceptical of anyone who saw one gender or another as superior, because he reasoned every man was just a mutated woman and every woman an undeveloped man.
And so an interesting wish came to him, one that would allow him to dramatically change the nature of gender. I wish everyone could change sex at certain times, he thought. I wish once someone gets to puberty, their body's sex isn't set in stone anymore, and they can turn into the opposite sex for a while. In normal circumstances they stay in their new sex for 24 hours, which extends by another 24 hours every time they masturbate to completion in that body, or until their pregnancy ends if they have unprotected sex and get pregnant.
He felt a strange wave of sensations race through him, and could tell his wish was being granted, but wondered what was to happen...