man, I'm not sure I'd call it sexism so much as realism. we know Jim plays pranks on Dwight all the time, Pam usually does the more harmless stuff, and she's known about the relationship longer and felt bad for Dwight etc.
it's actually one of the better shows as far as gender goes I think, as well as race! part of it though is because they have Michael make an insensitive joke or something and everyone cringes.