That article was really interesting, thnx 451.
Tentacle porn and the appeal of it is really complicated, at least from an outsider's perspective (it's not really a thing where I come from). I started thinking about it and didn't stop for a long time after I read these two posts. But really, if I had to simplify it, I'd guess that it's less about the tentacles standing in for human genitalia and more about how difficult it is to localize female sexuality (which has always been represented and believed to be less genital than male sexuality in the majority of cultures I can think of), and so it's difficult for a male to stimulate the diffuse erogenous zones that females have with only two hands. So the octopus can do it for him, like Zhao says. And he can watch. I'm not a voyeur so I can't really speak to why the watching or the visual representation is important.
But yeah there's that old "women=the sea" and also texture of octopus=female genital tissues thing.
(Actually, I love calamari and had some really good 7-day marinated grilled octopus at a chi-chi Greek restaurant once.)