Fans can be very flip and sophisticated about golden showers and whatever else in the name of artistic license when Jack and Ennis are paired. But let's face it, when Ennis engages in sexual experimentation with a new partner in life it is amazing how all that sophisticated offhandedness disappears to be replaced by schoolmarm primness and tut tutting disapproval!
While I share your bafflement over the reaction to events in the LS, it's not true that all other fics are given a free pass to do anything. At this very moment on DC there's an approaching-heated discussion about people's dislike for something going on in one of Maggie's fics. A lot of people take issues with things that have happened in "Beans & Crazies." There are a lot of fics that have gotten the "out of character" accusation or that things have been taken in directions sexually that are hard to believe. Nowhere near the reaction to the LS, of course, and I think you're probably right in that if it were Jack, it might have been okay.
And the rules are somewhat different in AU!AU. The LS and other fics that follow along after canon (like Riding Fence, for example) are often held to a stronger standard of keeping the characters in character. I don't believe that LS!Ennis is out of character as Louise has developed him. Some people do. Plain old ordinary AUs (like Leslie's fic, or mine) also are expected to hew closer to the canon-character line, although we get more leeway with taking the story in a different direction. AU!AUs, while most people feel that you ought to find some essential core of the charcter and bring it into the new surroundings, get the most latitude. A lot of character differences can be imagined for a Jack and Ennis that grew up differently, in different circumstances.
I'm saying that what might fly in an AU!AU might not fly in a post-canon fic.