If it has the adult flag, your book doesn't come up in searches. The adult flag has more to do with your blurb than cover than the genre. It's amazon's way of keeping their search results "clean," so to speak.
I've seen this happen where I'll search for a specific author and not have the book I'm looking for show up. I have to search for the exact title and author, and then I get a screen saying -your search result contains adult content, blah blah, click this if you're sure you want to continue... (and Amazon doesn't "remember" when you click this, so you have to do it every time). In other words, the book becomes invisible for all intents and purposes. (It doesn't come up when people browse the genre, search for key words, etc.--the only people who see it are the ones already looking for it.)
Erotica is a genre. 50 shades is "erotica." It doesn't have the adult flag