Burn
He didn't know why it continued to burn even after Orochimaru was long dead. He had killed his brother and returned to Konoha, attempting to rebuild his clan and get his life back together, but the curse seal would never let him forget the past.
At its worst, Sasuke found himself sometimes slipping back into old patterns from when he was training under the snake sennin, making him act in ways he didn't always want to. The heat on his shoulder made him aggressive, usually happening while fighting, or when he was with her.
Making love to her would sometimes trigger a reaction, the stimulus and lowering of his guard when he was caressing her naked skin allowing a power out that he normally repressed.
It made him want to dominate her, to force her to submit and suddenly he found himself needing to hear her recognize his position as master with trembling lips.
If she was on top, she was pinned under him in moments, if she was already on her back, he would thrust into her harder, almost animalisticly until he could see the fear in her green eyes even as her shuddering breath came in heady moans. She could struggle all she wanted, he liked it when she did, pink hair flying as she tried to get away from the darker version of himself, but she never succeeded and it usually ended in her sobbing out his name in ecstasy as he ruthlessly brought her to her pinnacle.
When it was all over, the burning would fade, leaving him to cradle her and whisper apologies. She always forgave him. She accepted every part of him, but Sasuke couldn't help but wonder what would happen if he one day couldn't return from the darkness he sometimes slipped into.