Suppose Karliah and Gallus' relationship while Nightingales wasn't just a coincidence. For Nocturnal's amusement, two of the Nightingales are supposed to be screwing.

Karliah actually picked Brynjolf because she thought he and DB were already having sex.

When she finds out they aren't, she suggests they start. Promptly.

http://skyrimkinkmeme.livejournal.com/1639.html?thread=626791#t626791