In most places the legality of incest is not dependent on the willingness of the parties. You are thinking about rape. Incest is not dependent on age, except for penalties if one party is much older than the other.
The term, "separation of church and state" is not in the USC, as you imply, no matter who coined it. the thoughts of private individuals regarding the same has no bearing on the letter and rule of the constitution, no matter who they may be.
