I think Simon is a bit worried about getting oil on his special parts. Still a class issue going on.
I think it is a class issue for both of them, actually. Kaylee totally has an inferiority complex when it comes to Simon's class. And neither one of them gets the other one. . . In The Train Job
when Simon asks Kaylee what they are doing, and she describes the crime and says "easy as lying," he looks at her like she's an alien, because lying does not come easy for him --
evidenced by how bad he is at being undercover.
And when Simon tells her that being proper matters more when no one in the Black cares, Kaylee doesn't really get it.
In the Simon/Kaylee follies, unlike most people, I generally side with Simon. Kaylee has a script running in her head about how things are supposed to go, and not only is Simon not on the same page, he's in totally different play most of the time. So instead of just telling him what she wants, she gets mad, flounces off, and he has NO Clue what just happened. Even though it's clear that they like each other, I just really find that Kaylee is unfair to Simon a lot of the time.
"We gotta go to the crappy town where I'm a hero!" -Wash