In which Harry realised he wasn't who he thought he was, and Draco was the one that showed him. (Harry is really a girl, and the wizarding world implements marriage contracts).
Harry didn't know why he wasn't particularly attracted to girls. He tried to attach lustful thoughts to the lumps of fat (commonly known was breasts) like most boys his age. But he just saw them as things that would assist with reproduction and feeding babies. He tried to focus on the curves of women and feel a jolt of interest, but all he got was artistic passion of sorts. Nothing sexual. He had never viewed this as a problem, he had decided that perhaps, he was just asexual. Perhaps, he just wasn't attracted to people.
That thought hit a roadbump when after he left school and continued his auror training, seeing the flexing muscles of the men around him caused his heart to pound. He figured that maybe he was growing tired, and his brain wasn't working properly. That was until he was in the changeroom and suddenly as he saw men changing in front of him, his body started heating up, reacting to the images around him. The niggling in his stomach he got around guys, that he always imagined was just his happiness, suddenly made sense. So, Harry started closing himself off. He was scared, he didn't know what to do. He couldn't make sense of the situation. But his isolationism came to an end shortly after the ministry created a new plan.
The Ministry of Magic decided that in light of magical Britain's size, they needed more children. Among that, they knew that they also needed something that would enable all blood levels to be interacting. So they created a spell that would match people up according to blood levels, so that no purebloods would be put together, and that as many muggleborns as possible would be matched with people already within the wizarding world. That plan was enacted and wizard folk all over the UK got letters saying who they were obligated to marry. Most matches made sense, except when Harry Potter received a letter saying that he was obligated to marry Draco Malfoy.
He had panicked, he shut himself deeper in his house, not even letting his friends come over. He was frantic. He was not only supposed to marry his rival from school, but he was supposed to marry a guy. The same gender that he was attracted to, the same gender that would ensure that there would be no reproducing between them. He didn't know what to do. Then he realised another problem as he noticed the gender pronouns on the paper. They all referred to him as a she. He was most definitely not a she, he knew that! He had the body parts to prove it!
But through this he discovered a secret that was long forgotten within the wizarding world. In their lust for blood purity and keeping their lines running, some families had enchanted their bloodlines so that any females born would appear as males, that way, they could always keep their families going. Harry's whole life had been a lie, he was not a man, and his body had known it the whole time. Despite his mind thinking he was a man, his body knew differently and tried to show him. He wasn't attracted to women because he was one. And now, he had to find a way to reverse the curse (cause that's what it really was), and deal with being engaged to his childhood rival.