It's also considered embarrassing to be walked in on taking a shit. To me, that doesn't imply that mainstream society holds shitting as shameful or immoral, in fact just the opposite.

I would assume most religions do condemn porn but mainstream US society is not religious.

Mainstream US society is fairly accepting of porn. The fact that it's joked about is basically evidence of this. Nobody jokes about e.g. getting caught fucking children, which IS condemned by mainstream US society.

The point is anytime somebody talks about having a problem with porn, there is some kneejerk reaction about how he is just confused by an unenlightened puritanical society, as if he is incapable of deciding for himself whether or not porn is negatively affecting his own life.