No, it's not. It only seems like the quality's gone down since the seventies because there's a shit-ton more porn available. Like TV or anything else we've become so inured to the shit due to volume that we tend to overlook the quality, even celebrate it when we find it.

I'll wager that the amount of good porn hasn't decreased. It's that the amount of crap has increased exponentially.

This also explains the improvement in the facial attractiveness.