I believe that there should be an etiquette book/booklet given to every child when they reach the age of 16 so they know what is proper and what is not. I am not just talking about weddings, but EVERYTHING (including thank you notes).
Yes, perhaps there would be some exceptions. A friend of the family got pregnant about 15 years after her last child (no affair) so my mom offered to give her a small baby shower. Sadly, the baby was dropped at birth and died. She wanted to give the gifts back, but mom told her to use hers for the baby to be buried in.
It is sad that people don't know that there are things that you just don't do - like PPD's or B-Listing people.