That's what I thought re: romance but... whoa. I've now been writing (loosely) in it as a genre for heading onto three years and it's... mad. Really. It's a monster, frankly, and I think there are very odd distinctions between what *is* romance by its actual nature (i.e., strictly a story about a romantic relationship, which ends on a positive note) and what *can be regarded as* romance, i.e., has a romantic relationship in it, but is also a mystery/action thriller/fantasy etc. The whole vampires-and-werewolves thing (with a side nod to shifter fiction) is a case in point. These things go in fashions.

There are sub-sub-genres, different styles... the surge in ebooks has reinvigorated it exponentially. Largely, from my experience, there's no difference between erotica and romance, because romance is increasingly judged on its 'hotness' - viz., the bandying of terms such as 'erotic romance' and 'sensual romance', which still adhere (mainly) to the same rules as Harlequin's formulas. The only major differences are what publishers coyly refer to as the 'use of direct language'.

As you point out, there's been more and more graphic sex in mainstream books in recent years (Wetlands, anyone?), so I see it as much as changing social tastes and expectations as an erosion of boundaries. Publishing has seen some seismic shifts in the past few years, and they're not over yet.

I'll dig out the dixploitation blog, though, and pop it up over the weekend.