He wasn't sure what it was, but there was something about the look in Percy's eyes that overwhelmed him. Perhaps it was the want in them, mirrored by his own, though Cass didn't care what it was, as long as it was there and Percy didn't look away.
And then suddenly (or had it been a long time?) he could feel Percy clench around him, see the look on his face as he spilled on both their stomachs. Cass was not far behind him, so ready already that he thought he might burst.
His hands tangled in Percy's hair as he held on, his body shaking in tune with Percy's as he emptied himself to the point where he nearly collapsed on top of Percy. He was hot and sweaty and couldn't stop smiling as he let his body linger on top of Percy for a while, unwilling to let go of him just yet.