Good news. Even pornography approval increasing, which would be not that great in a vacuum, is a sign of the weakening of many unfortunate social norms when combined with the rest of the data.
Adultery is adultery is adultery. I would hope most Americans would agree, but I don't believe I have the luxury of being surprised if I'm wrong.
Considering the social-libertarian leanings that this poll suggests, I have a feeling that you would get different results if you asked about consenting adults in a relationship having sex with other people if they have the explicit permission of their partner. I don't know if the approval rate would be very high, but it would probably be higher than 9%.
You probably would, yeah. Honestly, I kind of wonder how much that would differ based on whether or not the question was worded to include the phrase "consenting adults", which has started to strike me as something akin to the "it's my First Amendment right!" of sexual morality in terms of "what I'm doing isn't a serious felony, therefore nobody can criticize me"-ism.
Of course, "It's my first amendment right" is usually a defense for things that actually hurt people/a poor attempt at avoiding criticism, whereas "consenting adults" is a defense against arguments that typically boil down to "it's icky" in less childish sounding language.