Did Chad kiss a pig and not Sonny?

Yes, Chad kisses a pig instead of Sonny because right before they have to kiss, Marshall drops a pig on a trampoline, it bounces into Sonny's hands, so she makes the pig kiss Chad. This is ironic because right before the kiss Sonny says, "I'll kiss you for real when pigs fly." You can see this on episode 18 "Guess Who's Coming to Guest star", aka Sonny with a Chad.