Could you expand on what you mean here, specifically wrt why this makes incest wrong (if that is what you are saying)?
Imagine not having one person in your life who is not sexually attracted to you.
I think having people in your life in a platonic way is very very important.
That's one of the ways family is so important. If you are in a place where you can not move from the place with your family. All you want is parents or siblings who will be your friend and look after you. Sexual attraction usually clouds judgement. And weird power dynamics will come into play.
I do think that what is natural, is not such a sedentary/bound family structure, when the sex's are equal and there isn't so much possession it is usual for there not to be so much fuss on who the father is. So it's not a clear cut, this is right/this is wrong type of thing.
I think naturally if we have a healthy sex life (which most of us don't tbh) we have people who we are not attracted to, naturally there is many reasons for this but one is, close in dna, we want to breed with people who have different structure to us, so our offspring are more likely to survive.
The fantasy often comes from a love-place. If there isn't many people you have felt such strong love for and your sex life isn't healthy, we will often mix the two together.
You can have sexual fantasys and not project them onto people, you can feel the difference to one that will respect boundaries and one who won't.
nothing is ever 'wrong' or 'right' , but is this healthy? does this feel 'right' or is there apprehension? am i taking advantage of someone elses vulnerability? is this helping me grow? or keeping me stagnate?
i think when you say you can't do something or something is wrong it will always make people want to do something more
so whatever you feel you feel
but we must asses what we are feeling not just say 'this is how i am'