I don't really think it is taboo; I mean, you might get a few looks if you talk about your habits to strangers, but that's pretty much for the same reason that you'd get those looks talking about masturbation, even if you don't actually happen to be doing that while watching porn. Yes, there are strong ideas about the correct methods of sexual expression, which is to say that anything out of missionary position in a heterosexual relationship is outside of total societal acceptance, but porn isn't particularly unusual in that respect; a pretty large amount of people watch it, and it's so far from any essence of being underground that taboo is too strong a term. It's not talked about insofar as it's personal, not that it's disgusting per se.
I think straight porn, I guess the most common kind, is painfully misogynist; the man is either portrayed neutrally, without a personality, or positively, i.e. dominant (outside of s&m ideas), whereas the woman exists to serve and be almost robotically sexual. I don't think this is an inherent thing within porn; though it's often blamed for creating skewed ideas about women, men and sex, I think it's more accurate to say it serves an existing demand for this sort of emotionless, instantly and superfically gratifying portrayal of sex. If people were more comfortable with female sexuality, and a generally accepting and healthy attitude to one's own, then porn would be a lot more representative of real sex. Women would start watching it more, and the demand would change it to something more believable and engaging rather than the crass, irritating stuff we have now. You can find things like that, but it's a lot less common than what you usually get.
That all said, you can't beat a lesbian 69. And though one of my viewpoints sees it as an extremely complicated and interesting representation of what we all think about sex, sometimes it's really just as simple as that.