Good idea.
There is no fundamental reason to accept an axiom, as that reason would then be the axiom. Instead, axioms are defined when the overwhelming majority of people who understand the axiom accept it to be true without further reasoning. For instance, in mathematics, in any formula using variable
x, the statement
x=
x is axiomatic. There's no particular, deeper reason for
x=
x, it's just a useful rule to follow. (Although, it technically depends on the definition of the = sign - more
here.) It is useless to base your argument on an axiom that the opposition does not agree with.
As it stands, if you live in a democratic society - or at least, one where general cultural views shape the law, which is pretty much all of them - you will find yourself needing to deal with whatever everyone else thinks is true. You could have all the logic and science behind you; the fact is, everyone else still disagrees with you, and the opinion of millions takes precedence over the opinion of one. Seriously, I would imagine the percentage of people in, say, the US who actively wish for incest to be legalised is so small it rounds to zero. It's not going to happen in our lifetimes.
However, and as has been mentioned before, the illegalisation of an act is not necessarily a measure to totally prevent that act. Take piracy, for example - yes, it is objectively illegal to stream licenced anime from an unlicenced website, but you will never face any real repurcussions for doing so. Anti-piracy laws exist so that the state (or a corporation, or individual) may, if they wish, take legal action against someone committing piracy. Nobody is taking little Johnny to court for watching Naruto online, and no jury would convict him. Similarly, in the rare case of a truly victimless instance of incest, there is simply no reasonable series of events that might result in one being punished. If nobody involved has an issue, then it's not in the state's interest to care. The only reason anti-incest laws even matter is because incest almost always involves a victim of some sort, due in no small part to the unlikelihood of being legitimately attracted to a family member.
TL;DR: Truly ethical incest is effectively immune from law enforcement, and the argument for legal incest is irrelevant if virtually nobody will ever agree with it.