Harry Potter and the Magic Mirror
HP fan fic smut with Harry at the center. Several other pairings (and larger groups) are in there. Story is still ongoing but it's 15 chapters so far.
It contains the first time Harry and Draco are together, a few chapters in. Though this isn't (quite) their first sexual situation. Every chapter is sexual (smutty).
https://archiveofourown.org/works/5347841/chapters/12349127