Should parents be barred from sitting in on school sex ed classes?

They are in this school district (where parents are otherwise free to sit in on classes). I understand the reasons given for the ban, but my mind is currently affected by the script of the "Proper Condom Use" episode of "South Park" (which I read yesterday when I was writing about Laura Bush's milking-a-male-horse joke). In case you can't bear to read through the script (which you might -- should -- find awfully disturbing), I'll quote the moral of the story (spoken by Chef):

Look: Schools are teaching condom use to younger and younger students each day! But sex isn't something that should be taught in textbooks and diagrams. Sex is emotional and spiritual. It needs to be taught by family. I know it can be hard, parents, but if you leave it up to the schools to teach sex to kids, you don't know who they're learning it from. It could be from someone who doesn't know [a shot of Mr. Mackey], someone who has a bad opinion of it [a shot of Ms. Choksondik looking around], or even a complete pervert [a shot of Mr. Garrison].

UPDATE: The Washington Times gets results!