Though it does not thrill me to say so, you make a valid point.
It is worth noting though that cultural mores don't work well with shades of grey- they are best suited to black-and-whites. (i.e. do/don't instead of do this way/don't do that way)
Do you really feel bad about a consenting man and woman having a relationship? (or man and man or woman and woman) I have trouble identifying with this feeling.
With freely-available contraception and abortion, it's hard to think of any good reason for the absolute cultural taboo against all incest.
One of the goals of literature should be to challenge cultural taboos. Any good bookseller should realize it.