I've stumbled upon some genuinely engaging stories that weave adult content into the plot more organically than you'd expect. It's less about finding a specific 'scene' repository and more about identifying works where the erotic elements feel like a consequence of character dynamics.
Take 'Nozoki Ana' or 'Velvet Kiss'—both are manga, not anime, but the visual storytelling is so fluid. The intimate moments arise from intense emotional build-up, not just thrown in. For animation, you'd have better luck with certain OVAs attached to mature romance series, like 'Yosuga no Sara', where the atmosphere and character tension lead into those scenes. The smoothness comes from the narrative groundwork.
Honestly, most dedicated 'sex scene' compilations online sacrifice story for the spectacle. You're better off following a complete series known for its mature themes and letting the plot carry you there.