Oh. Oh, God. The sight of the Doctor tensing and arcing as he came, the sounds he made as he did, the feel of strong muscles clenching around the Master's cock... His orgasm shot through him, joining them as one for that one brief moment that stretched as long as he could make it, grunting and pushing into the Doctor as far as he physically could, not wanting it to be over.
Eventually, he couldn't make it last any longer. He exhaled, relaxing at last, and slipped out of the Doctor, reaching automatically for the tissues to clean them both up a bit and then collapsing onto his back beside the Doctor, looking sideways at him.
"Thank you," he said, meaning it more than he felt he'd ever meant anything.