Amazon started to exclude adult material from their sales ranking. Only that they define adult by gay/lesbian since mainstream het porn like that is still in the ranking but self help books and inexplicit gay romances are pulled from the ranking.
This is what you get if you search for homosexuality at amazon.com at the moment.
More information is here and here
Here's a petition
and this way to the google bombing amazon rank
but I think the most effective way to get them to react to the uproar is to pull your orders and write them as a customer.
ETA: Looks like the fixing is in process, some of the books I had searched for earlier today too have gotten their ranks back (Like "The Filly", by Mark Probst who was one of the first people who brought this up).
Hopefully a proper explanation and/or apology is coming too.
ETA: tehdely 's troll theory sounds not totally unplausible (especially since the regular porn was left untouched). Looks like several claimed responibility for the maybe hack in response of the theory. The internet is a messy place sometimes.