IMO, you're getting downvoted because your judgment of people on AM assumes facts not in evidence (i.e., that users of AM are there to cheat on their unknowing spouses), and then judges those people based on those facts that only exist in your head. It's self-aggrandizing, and the tone doesn't come off as participation in a discourse about morality as much as a narcissistic statement about the identity you'd like people to assign you ("wow, duncan_bayne is hardcore judgey about those AM users. He must really be against adultery.") See also [0]. Finally, it's also a derail from the reasons people are discussing the story that is more HN-relevant (technical and security aspects, user data practices, etc.).

So to answer your closing questions - 1) "no", people discuss morality all the time on HN, especially in relation to economic relations, 2) Probably "yes", because your judgment says more about what you want to say about adultery than about the actual behavior of AM users (who could have all sorts of non-adulterous reasons to be on the site), 3) clearly "no" (see (1)), but jumping to that overarching assumption that people are downvoting your comments because they just don't like morality is part of that self-aggrandizing impression I'm talking about.

If this was a dump of Petsmart's customer database, and you posted a bunch of comments judging the Petsmart customers as deserving to be exposed because they bought their pet from a puppy mill and some PETA links, people would downvote you, too--partially for the substance and partially just for derailing the conversation.

[0] http://www.davidbrin.com/addiction.html