This trainwreck of a thread is a pretty solid counter argument to the "people can enjoy a bad thing in fiction without normalizing it in real life" argument. People will look at an extremely romanticized fictional portrayal of something and then turn around and act like it's at all representative of the reality of that situation. Also, good god stop comparing incest to LGBT relationships you absolute loons.
Others have defended the moral reasoning, but I think there is another thing that needs to be pointed out.
In general we haven't been comparing incest to LGBT activity directly, but have actually been comparing (irrational) prejudice against incest to (irrational) prejudice against LGBT behaviors - the prejudice is largely unfounded, so comparing it says very little about the comparison between the targeted behaviors.
There are common themes that can be found, but they are at the most fundamental level - about privacy and freedom of choice, and apply to a whole range of human interactions.
Finally with respect to normalisation, I can't speak for everyone, but I have a long standing interest in ethics, and a fondness for stories of forbidden love, secrets, unusual passions and love in unexpected places. I on the lookout for thoughtful, well written incest romance stories years before I ever found any (among other things, I'm not just interested in incest).
By looking at "best case" stories where incest isn't a problem, we can come to better understand what the real problems are with the bad cases... you may see incestuous sexual abuse, but surely the problem is incestuous sexual abuse.
Look at the BDSM community - you see discussions and deep understanding about consent there far beyond the norm - because they play on the edges, assumptions and generalisations aren't good enough - they have to deal with the reality.
1x1/2 is a rare jewel - not only does it take the issues seriously, it is well written with likable characters and a light self-deprecating humour that makes it a pleasure to read.