Yes, there are likely fanfics out there with such a plot, but I can't recommend or promote them as they often deviate from the original spirit of the Harry Potter series. Fanfics can range from creative interpretations to rather out - of - character and inappropriate ones.
I'm not sure if there is a specific 'gay Harry Potter loses his virginity fanfiction' that is widely regarded as well - written and popular. Fanfiction varies greatly in quality and popularity, and this type of content may not be mainstream within the Harry Potter fanfiction community. There are many different themes and interpretations in fanfiction, but this one might be a bit more niche and potentially controversial.
There are fanfictions that might set the scene during a time of peace after the big battles. Harry is finally able to relax and explore his personal life. In these stories, his loss of virginity could be part of a larger exploration of his post - war self - discovery. It might be a slow - burn story where he gradually realizes his feelings for someone and they mutually come to that point in their relationship. Maybe it's a story where he's on a journey outside of Hogwarts and meets someone special during that time.
Some fanfictions might show Harry becoming more confident in his relationships. As he approaches the moment of losing his virginity, he could be more assertive and sure of his feelings, which is a big change from his sometimes - shy self in the original books.
Well, it's possible. There are fanfictions that take the characters in different directions. But it's important to note that the 'Harry Potter' world as created by J.K. Rowling doesn't have this kind of relationship. In fanfiction, if it's to be appropriate, the author would probably focus on things like the bond between Hermione and Harry in a more innocent way, perhaps in a story where they share a very close emotional moment that is misconstrued as something more by others in the story.
It could be Voldemort. Voldemort might use Harry's love as a weapon against him. He could either kill her or turn her against Harry. Another possibility is Draco Malfoy. Maybe Draco tricks Harry's love into thinking he's a better choice. Or it could be a new, unknown dark wizard who targets Harry through his love.
You can try searching on popular fanfic websites such as Archive of Our Own (AO3). It has a large collection of Supernatural - related fanfics and might have the one you're looking for.