I'm noticing the platform landscape for mature Fairy Tail content has shifted over the years. FanFiction.net has a blanket ban, so you'll only find works there that 'fade to black.' Archive of Our Own is the central hub now because its tagging system allows you to filter for explicit content and specific pairings, which is essential for your request. The tags 'Nalu' or 'Gajevy' plus the 'Explicit' rating filter will bring up most of what's current.
You have to wade through a lot though—quality varies wildly. Some authors focus entirely on the intimate scenes without much build-up, which can feel hollow. The real standout stories I've found are often longer slow-burn fics where the eventual lemons feel earned by the character development. Those are rarer. I'll sometimes find a gem linked in the author's notes on a tame fic, so don't ignore M-rated stories entirely as potential leads.
Tumblr blogs used to host a ton of this, but the NSFW purge a while back scattered that community. Some authors migrated their work to Ao3, while others use private blogs or discords now, which makes discovery harder.