I dunno know why they get so silly about it - it's not like you are asking them to change your childs butt!

However - I'm pretty sure they have no idea about MODERN cloth nappies, and most likely think about the old terrys and plastic pants. Even if you were I can't see why it should be something to be irritated about!
As Kim says - they will soon shut up when they actually SEE what you are talking about