No. This kind of description involves vulgar and inappropriate content which has no place in proper fanfiction or general discussion related to Harry Potter.
You can try fanfiction.net. It has a large collection of 'Harry Potter' fanfictions, and you're likely to find ones related to Harry's relationship with Sirius there.
One reason could be the complex relationship between Harry and Sirius in the original books. Their bond was deep, and some fans like to explore a different, more romantic or intense connection through slash fanfiction.
Well, first off, a great place to look for 'Harry Potter wants Sirius Black' fanfiction is Tumblr. There are many blogs dedicated to Harry Potter fan works. Some users share their own fanfiction, and others reblog great pieces they've found. Another option is Wattpad, which has a large collection of fan - made stories. You might have to sift through a bit, but it's worth it. You can use the search function and type in relevant keywords like 'Harry Potter' and 'Sirius Black' to narrow down the results.
One popular story is 'The Unspoken Bond' which delves deep into Harry's longing for Sirius's presence and guidance. Another is 'Sirius's Return' where there are some really interesting plot twists around Harry's desire to have Sirius back. You can find these on Archive of Our Own.
One common theme is Sirius as a father figure to Harry. Since Harry lost his parents early, Sirius can fill that role in fanfictions, guiding Harry and protecting him.
You can try looking on some popular fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you might be able to find ones related to Harry Potter and Sirius with the 'slash' theme there.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.
One great fanfiction about Sirius' childhood could be one where it delves into his relationship with his family. Maybe it shows how he started to rebel against the pure - blood mania in his family at a young age. For example, he might have befriended a Muggle - born wizard secretly, which his family would have abhorred.