Sam will live if he knows his big brother slept with an archangel. It might be knowledge and mental images he doesn't want, but he'll survive.

"You realize that I'm not exactly keeping tabs on what's being shared with him and what's not, right?" Gabriel ask rhetorically before shaking his head. "Things are fairly calm Upstairs, last I heard. Had to smack down a couple of people for getting over-zealous on the whole 'make everybody go back home' thing along with the angels that objected strongly to going home."