See, when I was but a young teenager, I really liked slash (ah sex - it's an intriguing mystery for the young!) - so I was a lurker in fandom and discovered all the sane bits myself (and more importantly, discovered that I actually did favour plot over porn). So, I was lucky in that I was drawn in by shows where the main characters WEREN'T related...so I knew that there were parts of fandom that DIDN'T do the porn thing when I joined fandoms where characters WERE related. (Because I don't care how much slash I used to read - incest just crosses a line into squick.)

...and yeah, as you say, once you see the horrific stuff, the tame stuff really is practically normal.