Because Quinn thinking Brody's hot is an entirely valid reason for refusing to kill him last season, and everybody knows that when an assassin refuses to kill someone, that sexing must ensue. It's practically a law.
Also, I have an unrelated question. In last week's ep, Carrie discovered she'd been stitched up in the morning paper and went immediately to confront Saul - who appeared to be just finishing a several-course meal in a restaurant. I mean, I can admire a man who eats tiramisu for breakfast, but it did seem a little odd. Had he been there all night? Had it taken her all day to find him? Did I miss something? (entirely possible).