Ianto had gone undercover as a priest to defeat an alien that looked like a demon. Jack had defeated the alien which once killed had exploded into green slime covering Ianto and everyone else in the room. Depite the green alien slime Jack had a very inapproiate reaction to seeing Ianto dressed as a priest.
"Jack your mentally scarring people," Ianto complained.
Jack lowered his voice. "Don't worry I'm going to retcon them later anyway. You know after you get that outfit cleaned we could have a lot of fun with it."
For some reason I can totally imagine this happening.