We don't call 'em "private parts" for nothing.

The basic, fundamental dynamic of sex is intimacy. Sex is, at a basic, sociological level, about sharing privileged body access. When you strip the sociological apparatus of sex down to its bare essentials, everything about sex is about sharing something that you don't share much. The negotiation of sex is a negotiation of sharing - the brokerage of a shared taboo. The only societally sanctioned sex is that within a marriage which, up until very recently, was seen as a vessel for procreation. All other sex, then, is an illicit deal for morally proscribed recreation.

Thus the cultural baggage associated with pornography - the consumer gains proxy access to this taboo. The producer devalues the sanctioned act. And that's just within the social framework - take society out of it and there's still a pregnancy/disease risk to contend with. From any angle you choose to view it from, sex is a protected transaction and pornography is bootlegging.

That's part of the allure of sexting or taking dirty pictures - you're "being naughty" with someone else and that will always appeal. But like you said - it's a private transaction and part of the negotiation. Part of the dance. You aren't "selling out." It comes down to intent - are you sharing photos to increase the intimacy? Or are you a mercenary looking to pad the bottom line?

I wouldn't blame society. I wouldn't blame "the patriarchy." Pornography takes an exchange and turns it into a theft. If it didn't, it wouldn't be pornography.