Moral preconception or moral dumbfounding refers to the determination of all possible rationalizations for an ethical position by preconceived moral fiat. Jonathan Haidt, for example, shows how most people will find a reason to condemn incest on the basis of no contraception, and then pivot to another rationalization once told that contraception was used. They are said to be led by their moral principals, but prefer to find a rational justification.
The phenomenon is also present in arguments against minor-adult sex. It is typical for debaters to argue that the supposed burden of harm spanning from these relationships makes them intrinsically wrong. When presented with evidence to the contrary, they will often pivot to moral arguments led by negative utilitarianism, power imbalance or supposed neurological trends.