If this was completely intentional, it wouldn't be such a big deal. Non-political censorship is a minor matter. It would be terrible news if Amazon was de-ranking liberal or conservative books. But this is a minor issue. Worst case scenario, men will have to go back to jerking off to Sears catalogues. How difficult is it to find "adult content" on the Internet? My guess is that 90% of people trying to buy adult oriented books have a pretty good idea of what they're looking for. They probably have a name or saw something online. Otherwise you might buy something that includes she-male porn. Or does not include she-male porn (I didn't want to leave the Apple guys out).
Gay and lesbian books are a niche market. So are many other things. Dan Brown novels and Harry Potter books appeal to a general audience. Not saying that they're great, just that a wide scope of people buy them. Pregnancy books are an example of a niche market. People who want to find pregnancy books, search for pregnancy books. They are completely irrelevant to other people. The same for deck repair. Most non-gays are not interested in gay material. Joe Hetero-Average or Jane Hetero-Average do not benefit from getting gay book hits on their generic searches. If you want a gay cowboy story, search for "gay cowboy". If you want a book on kitchen repair, search for "sink disaster".
If this wasn't a glitch, it's because the vast majority of the world's population is repulsed by homosexual sex. The same way that the vast majority of the population do not want to see a man and a woman who have a BMI of 50 have sex. The media image of sexy lesbians is pretty much lesbian blackface, they are usually presented as two straight women who are having sex to arouse a man. This does not mean that people want homosexuals imprisoned or punished, the public just doesn't want to watch them "exercise" their freedom. Just as most people do not want to watch a burn victim have sex with someone with a colostomy bag.
As far as sales figures go, I've never understood why people are so likely to follow them. I don't have grey hair and I've known for a long time that sometimes half the people in the country buy drek. Popularity and Quality are independent of each other.