You might as well claim it's unethical if you buy gold and then someone finds a gold asteroid and makes the price drop.
The ethicality of an action is derived from its motive. The very same action can be ethical by occurring from one particular motive, and unethical by occurring from another. E.g. killing another person - in self-defence, it's fine; from wanton aggression, not (an involuntary contract).
So if we see a central bank printing money because it wishes to obtain our wealth for its own use, this is unethical; it is theft, an involuntary contract.
We see also central banks apply an involuntary contract, using the force of law, to mandate their currency is the only legal tender in the nation.
OTOH, if someone goes out and finds a new gold asteroid, they're not doing this to obtain wealth from others; they're doing it to find new wealth. I also note it likely takes a good deal of time and effort and risk on their part to do so, and that the vast part of the wealth they obtain is derived from obtaining *new* gold, e.g. there is a genuine increase in real wealth, rather than by the devaluation of existing gold. Indeed, the devauation of existing gold *is of no benefit to the miners*, as it only acts to devalue the *new* gold they have found. It does not transfer that wealth to them; in fact, the opposite. This is unlike the central bank scenario, where devaluation of the currency held by others *is* the way in which their wealth is obtained by the bank.
However, despite these critical and fundamental differences in motive which clear distinguish between the two scenarios, we still do see in the latter scenario an increase in the supply and so the amount of gold and as such a proportional discouragement in the value of the gold, to the detriment of all existing holders of gold.
I think here we must think about the basic rule; all contracts must be voluntary and well-informed, except in self-defence.
I think the core issue here is that the holding of gold is a voluntary act. If I hold gold, I cannot then impose a contract on others by saying "*I* choose to hold gold, so *you* cannot mine any more".
This differs from the central bank fiat currency scenario by the range of contracts typically imposes by the State; there is only one legal tender, alternative wealth holdings other than gold are at least taxed at the rate of VAT, making them impossible chioces, etc. The holding of sterling is not a fully voluntary act, because many State-induced compulsions exist which make life extraordinarily difficult to those who live in the UK and choose not to hold any sterling.
If a central bank imposes itself upon the mass of people and then prints money, it is unethical by the compulsions involved. If a central bank however printed a currency which came with absolutely no obligations and an up front warning that the bank could print money and so devalue that which is held and people then *choose* to hold that currency, then it's okay. They knew what they were getting in to.
