Neither of them can really handle that.
Brett "Everyone must like me all the time or I will die" Hand, who vomited at the thought of firing someone, could not handle working alongside someone he's banged. Clingy as hell. Unlike Rafe Masters, he wouldn't immediately start assuming a deeper relationship, but he would alternate between withdrawing into a quiet shell of sadness about not being together and awkwardly hinting at starting a real relationship.
Reagan, meanwhile, can have a one night stand and leave it at that, but not if she has anything like positive feelings towards the guy. Rafe was easy because he was exclusively a shallow attraction. With Brett, I don't think Ms. "No serious boyfriend since age 15" has the practice in separating romantic and platonic feelings after sleeping with someone, she'd throw herself into her work and hope that distracting from the formless void of feelings, because if she still likes someone and slept with them, that means she loves them, right? right?