Yes, if Clinton argued in favor of chastity, those attacking chastity, if they were intellectually honest, would present arguments against chastity, not arguments against Clinton.
People who say they are arguing against Buchanan's position, but really only argue against Buchanan, thereby confess that they are unable to argue against Buchanan's position. Which is why the argumentum ad hominem is generally frowned upon, at least by those seeking truth.
You're drawing a nonexistent distinction. If some guy were going around selling "Dr. Lighning's Miracle Elixir" as the cure for all ills that afflict the flesh, and he was seen checking himself into a conventional hospital to cure the ills of his own flesh, this fact would certainly be relevant in deciding whether or not to trust the elixir.