Well see, it could go a number of ways. It could be that Sam IS infected, or that the other folk make a run for it instead of coming back, and there has to be shooting, possibly after sex (definitely not prior to, I think David Cronenberg manages all that wound-fucking quite smashingly all on his own), or yaknow, you could always go the simultaneous/snuff route. OR, it could be like canon, Sam is NOT infected, and the other folk just decided to "whatev! sweet ride! let's just take the car and go!" So Sam and Dean are off having trashy/gritty/sweepingly romantic sex-that-involves-medical-stirrups, then have to kind of wait around awkwardly until it becomes apparent that whoops, Sam isn't infected after all. hah hah. Hah. Hoo, boy.