I actually didn't think of Spike and Buffy. Now that you mention it, though, it does kind of make me wonder if that's where they got the idea. I mean, the Spike/Buffy scene is officially the hottest TV sex scene. I can see a movie producer wanting to convert that mojo to the big screen.
That scene in Mr. and Mrs. Smith cracks me up. I like the part in which Mr. Smith kicks Mrs. Smith repeatedly. The look on his face is hilarious. You know, honestly... Mr. and Mrs. Smith made me laugh while they were fighting and then comshucking. Buffy and Spike made me go, "WHAT? Is this really happening?" lol. Two completely different emotional responses were elicited. I guess it's because I went into Mr. and Mrs. Smith sort of expecting that kill me/kiss me vibe. I went into that episode of Buffy expecting freeze rays and rat girls.
It might also be different because I knew that was what was going to happen in Mr. and Mrs. Smith. It was a movie starring Brad Pitt and Angelina Jolie. It would be sexy and have a happy ending. I kind of worried about Spike and Buffy in their fight scene, though. I really thought one of them might die (again). I actually thought it might be Buffy, who would then be revealed as some sort of immortal demon (because of Spike's chip not working). Then, I thought that might result in Spike getting awfully dusty. So, yeah. Completely different vibes. In one scene, I knew they'd end up having sex. In the other, I thought they might really kill each other. The one in which I thought they might actually kill each other was much more suspenseful.