Amazon is on a mission to flag adult content and keep it out of the general audience search engines. This is not necessarily a bad thing, if done correctly. Honestly, I don't want nine year olds reading even the excerpts to my books.
What it means to you, however, is that, as reader, you will not see any adult material in your search -- UNLESS you enable your account for adult content.
Here's how I did this:
Select Kindle Store in the search drop down. Search for the title of an adult book (In my case I used Cuffed by Megan Slayer). You'll receive the following message:
Your search contains adult items which have been hidden. If you wish to see them, click here.
Once you enable adult content on your computer, all the books will show up again.
(At least until you clean out your cookies. If you're using a browser cleaner, tell it to leave Amazon's cookies alone!)