One of our past threads, though a bit old (so some answers wouldn't really be acceptable in 2015 /r/AskHistorians):
This is a very ahistorical question. You are assuming that gentialia did become universally seen as "obscene" at some point in history. I'd argue that this is not the case. There have been some cultures and periods in time where they were seen as such, but it's hardly a universal concept.