Just as a reminder, Douglas Harper is a decent enough guy but not a reliable source. He just cribs other sources, often badly.
Hardcore is obviously a compound of hard + core and, as you'd expect, it shows up first (early 19th century) in reference to building material with a literal "hard core" but no other distinguishing features (=rubble). By the first decades of the 20th century, that had become a single word and then started being used more broadly. The ''OED'' gives a 1916 use in the ''Burlington Magazine'', No. 29, p. 39: "However good socialists we may be, we all have, I believe, a hard core of anarchism within us..." The 1936 references to the "hard core" of unemployables who shouldn't expect to be redeemed by New Deal social programs describe the term as already in widespread use.
The sense you're complaining about is all from porn, though. It was originally an obvious pun, but has seen greater use as sex and drugs and rock'n'roll stopped being transgressive and simply became part of the mainstream culture.
[Trivia: As far as the OED knows, the first use of hardcore in reference to music is a Canadian who was praising Jamaican artists who would slip their discs into David Bowie's covers in the late '70s.]
