Some fanfics use it as a plot device. Maybe there's a spell gone wrong that makes Harry attracted to girls' clothes. This then sets off a chain of events where he has to deal with the reactions of his friends and the wizarding community at large. It could also lead to him learning more about acceptance and individuality.
It could be Hermione. She might notice that Harry's clothes are looking a bit shabby and use her cleverness to either buy or create new clothes for him. Maybe she uses a spell to transform some old fabric into a brand - new set of clothes.
Maybe it's for a school play at Hogwarts. The teachers might choose Harry to play a female role in a play that has some important significance in the wizarding world's history or mythology. This would be a simple and innocent reason for him to wear girls' clothes.
One example could be a fanfic where Harry discovers the comfort and unique styles of girls' clothes during a magical accident that makes him temporarily see the world from a different perspective. In this fic, he might start wearing a girl's cloak and find it much warmer and more stylish than his usual ones.
He might get a new wizarding robe with intricate magical patterns and symbols that represent his house, like a Gryffindor lion motif in golden thread on a deep red robe.
There should not be any such fanfiction. The Harry Potter franchise has a wide range of positive and creative fanfiction, focusing on things like character development, new magical adventures, and relationships within the magical world in a proper and respectful way. This kind of inappropriate idea does not fit into the normal scope of fanfiction for the series.
Well, it might seem a bit odd at first. Snape has such a complex relationship with Harry in the original story. But on the other hand, fanfiction can offer different perspectives on the characters. Maybe he likes it because it shows different interpretations of the wizarding world he's a part of.
Well, it could be. Since the Dark Arts in the Potter universe are generally bad things like curses and black magic. Harry has always been on the side of good, fighting Voldemort and his Death Eaters who use the Dark Arts. So for him to like Dark Arts fanfiction might make some people think it goes against his nature. But on the other hand, fanfiction often explores different aspects of characters, so it might just be an interesting and perhaps controversial exploration.
Well, I'm sure there exists some. Fanfiction allows for a great deal of creative liberty. But when it comes to 'Harry Potter likes boys', it may face some criticism from more conservative fans. After all, the original story doesn't really hint at such a relationship. But in the fanfiction world, authors can take the characters and put them in various scenarios, so it's possible that you can find some works that explore this idea.