I totally get the curiosity about exploring adult manga, but I'd be careful with free sites—many are sketchy with malware, ads, or even stolen content. If you're into legit options, some official platforms like 'MangaDex' (before they cracked down) or 'ComicFury' host indie creators who occasionally dip into mature themes. For classic works, checking out publishers like FAKKU or Project-H might be worth it; they often have sample chapters.
Honestly, though, nothing beats supporting artists directly. Many Japanese creators have Pixiv or Fantia pages with free previews, and some Western artists share NSFW work on Tapas or Gumroad. It’s a rabbit hole, but ethical consumption keeps the industry alive. Plus, you stumble on hidden gems that way—like this one vampire-themed doujinshi I found that had way better lore than most mainstream horror manga.