Something can be 100% consensual and still be illegal: if 2 guys agree to an old-style duel with flintlock pistols or fencing swords, and one of them gets wounded or killed, the other guy is still guilty even if both of them unambiguously agreed to the duel.
And while it's true that (incestuous) sex shouldn't result in wounding, there is the *emotional* damage of incest to take into account, as well as the harm visited on future generations by inbreeding.
Something can be 100% consensual and still be illegal: if 2 guys agree to an old-style duel with flintlock pistols or fencing swords, and one of them gets wounded or killed, the other guy is still guilty even if both of them unambiguously agreed to the duel.
And while it's true that (incestuous) sex shouldn't result in wounding, there is the *emotional* damage of incest to take into account, as well as the harm visited on future generations by inbreeding.