Re: A Short Youtube Documentary that Reveals the Fate of Bitcoin Holders
I get your point, but, actually, even that example's retort is a fallacy, because the person arguing for marriage between a man and a woman is arguing that only the act itself of getting married in a different context involves sin, not that a separate sin can't be committed while a man and woman are married. In other words, that retort is no different than "Oh, really. So how come you murdered someone while you were married to your wife?")
Sidenote: this is heavily offtopic of course. But since OP opened a rather worthless thread, I don't feel bad for hijacking it
I get your point as well, however:
What you describe would be true in a sort of platonic realm of arguments, where each proponent of a position would be completely unrelated on a personal level from the position he or she presents. Let's say, how a mathematician would present a proof. He could be wrong, he could be right, but there is only the validity of the argument itself, and it has no relation to the individual presenting it other than that the person presenting the proof contains the brain that computed the proof (or made an error doing so).
I believe that, even in principle, that's not how non-mathematical arguments work. There is no "platonic ideal" of, say, the right amount of taxation. Or whether it is morally wrong or right to marry man and woman only, or man, man goat.
So my claim is: even when we can make some references to more abstract, possibly objective points in an (moral) argument, we will also inevitably invoke some personal aspects. Like "personal integrity", or "consistency of principles of ones own life".
And it is at this point, if there is a clear contradiction between one of the claims of the argument (say: "marriage is holy") and the personal example of the proponent of the argument (say: he cheats on his wife) that an ad hominem is a valid counter - not because it goes against any principle that the proponent of the argument mentions, but because the proponent inevitable has to rely on personal aspects to further his argument (in the case of moral/aesthetical/maybe also economical arguments).
Does that make any sense?