It's was a power thing nothing to do with incest
But incest is about power. That's part of why it is so disgusting. I would even say it is the main reason why incest is disgusting. If two siblings who never met before fall in love, that's a tragedy. If a man sexually abuses his adoptive daughter, it's a disgusting, horrible crime.
I don't much care whether a man who does this "I'm your daddy" thing wants to fuck his actual daughter, or just wants to fuck a woman he has as much power over as he would have or has over his actual daughter. It is both disgusting.
And for those saying that it's just an "I'm the best" phrase ...every normal person is disgusted by any mentions of terms for relatives during sex.
And also, "I'm the bestest" is just not something a man should say during sex. Even if he manages to phrase it without the word "daddy", it's a turn off.
Also agree with AskBasil, we should question this whole idea that sex needs to be transgressive.