Once upon a time, girls pleasuring other girls was unthinkable to me. I found that exciting because it was weird and different. Now? I don't find it weird or different. I work with a few people who identify as lesbian, and for the most part, it makes the "taboo" less interesting.
There's a romance to mystery, to the wrongness. It's instant. It should be wrong and icky. Why watch a Horror movie? Why subject yourself to a thrill ride roller coaster?
Some things heighten the senses because of what they are. They aren't meant for normal human beings, in normal life. The taboo of incest will always be this way, IMO, in a way that lesbian porn used to be for me until it became normalized.
You don't like it? That's fine. I find it makes the experience of a fantasy more exciting... not because I, personally, want this in real life. There are countless horror stories of real life incest-rape. I don't want to visit that.
However, weird and shocking in a fantasy? I love it.
Non-consensual sex? Another problem. Bestiality? Yet another problem. All of this is some degree of wrong in real life, but as a fantasy? You bet I like it.