in a nutshell: people are stupid. Particularly about sex.
Looks like a couple things are going on. There's the whole "please be my mommy cause I can't do it myself" bullshit, wherein someone at Amazon decided it would be a spiffy idea to implement a SafeSearch feature so nobody ever has to be bothered by accidentally being reminded that sex exists while looking for some other type of story ever again, thereby ghettoizing any book that has anything to do with sex into the virtual equivalent of a back room behind the counter. Hence, in order to, say, find a book on how to have a sex life as a disabled person, you'd have to know the title and author of the book already and specially request it.
Then there's the old conflation of "queer" with "sex," such that something like Heather has two Mommies or the memoirs of a transman or an academic consideration of the political and military ramifications of DADT are automatically interpreted as the equivalent of hardcore porn, because of course if it's about gay it must be about sex.
Hence why the YA and academic GLBTQ world has been the most glaring victim, but any number of other stories have also been ghettoized (from feminist lit to self-help and erotica).