A 14-year-old having sex with an 18-year-old is still statutory rape, with or without the law on incest. That doesn't change. We aren't talking about pedophilia; we aren't talking about age of consent; we aren't talking about statutory rape, or rape of any kind.

The median age of first sexual contact is 14, with another 14-year-old. It's a fair bet that when you were a minor, you had sex with other minors.

Two 14-year-olds having sex with eachother is not pedophilia; it's not statutory rape. There are no criminal penalties for two 14-year-olds choosing to have sex with eachother. Unless they are related by blood. Then, they can be sent to state prison. This is bigotry.

Two 24-year-olds, being well above the age of consent, there is no serious question that they are "old enough". They are capable of consent, and consent is granted. Completely inoffensive relationship; the state has no interest in intervening between them. And yet, if they discover they are biological kin, they are to be imprisoned. This is bigotry.

The pedophilic insinuations you are making play no role here. They serve only to distract. And yet you keep raising them, again and again. You insist on this unreasonable assumption that "old enough" doesn't actually mean "old enough", to distract and detract from the underlying issue. Engage in good faith, or don't engage at all. Discuss the same-age couples above. Justify the california law calling for them to be imprisoned. Justify depriving them of their right to free association. Justify stripping them of their personal agency regarding consent. Justify leaving their future to the whim of judicial discretion.

Or concede that incest should not be an inherently criminal matter.

We don’t assume people wanting to take part in incest are altogether reasonable, logical or for that matter moral so assuming any of those things is the unreasonable thing to do.

That is a rhetorical fallacy called "begging the question". Rejected.

you simply assume benevolence when there’s no evidence that that is the case.

In the field of philosophy, "Hanlon's Razor" is the most relevant concept. In law, it's called the "pro reo principle" or the "rule of lenity". Related, the idea of "innocent until proven guilty". We should, indeed, assume benevolence until malfeasance is positively proven.

Basically, reasonableness is the default state. We don't need any sort of evidence to assume reasonableness. We need evidence to demonstrate unreasonableness.

Here, that means "old enough" is, actually, what any reasonable person would deem old enough. OP is not making any sort of argument regarding the current age of consent laws. OP's argument is that, whatever society deems an appropriate age of consent, an incestuous relationship initiated above that age of consent (with capacity and with consent) should also be legal.

But again what happened to the rest of your argument and all those non existent fringe cases that are already legal in North America?

They certainly exist. Marriage, for example, has a wide variety of financial and legal benefits and consequences. Incestuous marriage is criminal. I don't need to provide a full, comprehensive list of all the ways that criminalizing incest affects every possible victim of anti-incestuous bigotry. Any social or legal distinction is sufficient to question the need for that distinction.