If the daughter is of legal age and it is consensual then I would support this as a relationship or just sex. We all have different views on what is acceptable and what isn't. If the father and daughter involved are happy with their situation then who cares. In some countries this is actually legal and it should be everywhere. Also, I have the same opinion for other family members too including mother and adult son, Adult brothers and adult sisters etc. We must have the right to choose.
In another hypothetical scenario, Say a man is raped (somehow, Drugging, Maybe), The woman who raped him has a daughter who goes to find him as an adult and they end up in a relationship. This removes the infidelity aspect present in nearly all instances of this relationship, Thus leaving nothing to take offence to. You can't say it's "immoral", Especially if you support homosexuality, And you can't talk about protecting them because you can protect people in a relationship.
Lets say you find a woman and are intimate with her only to find out that she was your daughter.
To any normal person that would bring great shame and regret. Even if you never knew that she was that act would make any person disgusted about their own selves and they would stop.
Weird distorted people would have no problem with that but every other person would stop on the spot.
I really support individual freedom and space. But regarding sex between close individuals is against the morality. The duty of the father is to protect the daughter and not to spoil her life. Incest will definitely affect the mental state of the daughter in a very bad way. This is really sickening idea.