This is all text from the homepage of Ashley Madison:
"Ashley Madison is the world's leading married dating service for discreet encounters"
Their tagline, a registered trademark is, "Life is short. Have an affair.®"
"Ashley Madison is the most famous name in infidelity and married dating."
"Thousands of cheating wives and cheating husbands signup everyday looking for an affair. "
You are conflating the marketing copy of the company with the intentions and motivations of the users. Just because something confirms your most cynical expectations doesn't make it true.
Ashley Madison has absolutely no idea whether any particular user, if they are married, is on there to do anything with their profile other than browse, or whether their spouse knows they are on there, or for that matter if a couple is browsing the site together as part of their joint fantasy life, or if they are in an open relationship, or a number of other scenarios that have no impact on you and really provide no basis for you to cast judgment about their motivations--especially in an absence of evidence.
See again, my cite to David Brin's article on addiction to outrage.
The reputation, marketing, and design of guns is a good bit more varied than Ashley Madison's. But yes, buying and learning to use a gun shows a certain intention or willingness to kill. It may be to kill a deer you're hunting or to kill a person in self defence, but in general it's evidence of a certain willingness to kill.
One could even have bought it out of simple curiosity.
I was speaking generally. I said that it's reason/evidence in favour of a conclusion. I said that it isn't proof of it. Those kinds of general rules and presumptions are useful for all kinds of things and in all kinds of ways. Or do you think that our judgements should be entirely composed of perfectly comprehensive and universal rules?
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