Basically Murdoc accidentally sees Mac and Jack going at it. Why he's there or how he sees are unimportant, just that he didn't expect it and than is too interested to leave and watches.
And gets jealous of Jack.

Bonus if its rough sex or flat out BDSM (with Mac subbing), because that would be unexpected to him and probably turn him on more.


I know this has actually been done already, but it apparently hits a "kink" I didn't know I had and I'd love to see more >.>