Clearly from the response we're seeing from society outside of this tech bubble where your privacy must be protected at all times otherwise doom ensues, people want to know when their spouse is cheating on them. You make an assumption that quite frankly ignores reality in favor of a weird moral relativism where any sort of judgment is bad.
Odds are that people don't want their spouse cheating on them.
Someone stealing from someone else doesn't have an affect on me either, but I would operate under the default assumptions that
1) they want their things
2) they would want to be notified if someone wanted to steal their things
3) they would want to be notified if someone actually stole their things
And guess what? People acting in this way has the prosocial implication of teaching people that stealing is bad, and gives me the assurance that if someone is trying to steal from me that people will let me know!
Hacking into a dating website is not analogous to merely observing someone stealing. If you set up cameras pointed into my bedroom, I'd have a big problem with that even if it did catch a burglar.
The fact that some people approve of vigilante justice and disrupting the personal lives of people they don't like doesn't mean it's a good idea. There are many obvious historical examples of terrible harm that sort of thinking has caused. If you think adultery is a crime that requires help from people not involved in the marriage, I suggest you campaign to make it illegal in all states and ask for police resources to be dedicated to it. If you see someone stealing, you should call the police.
Again, you have no way of gauging intent. An example the poster above gave is a really great one. A couple having a consensual S/M relationship that involves controlled pain, possibly leaving visible marks. Your method would be to immediately rat them out to the authorities. Congratulations, you just ruined a happy couple's relationship because you can't see past your own thin worldview, and never bothered to think before you act.
BDSM is legal under Lawrence v. Texas in the United States, as I recall. The police are generally aware of this. Though, in this situation, a reasonable person would ask first.
From what I've seen, interest in the Ashley Madison leaks has been almost entirely prurient ("OMG look at this guy!"), which is a common human foible, but not something that's generally considered good to encourage. Many people also reacted positively to the theft of compromising photos from dozens of celebrities last year, for very similar reasons.
Odds are that people don't want their spouse cheating on them.
Someone stealing from someone else doesn't have an affect on me either, but I would operate under the default assumptions that 1) they want their things 2) they would want to be notified if someone wanted to steal their things 3) they would want to be notified if someone actually stole their things
And guess what? People acting in this way has the prosocial implication of teaching people that stealing is bad, and gives me the assurance that if someone is trying to steal from me that people will let me know!