The last is quite complex and I won't get to it here because of the vast differences in moral theories as to what should and should not be legal.
As to the first, is it legal? Hacking isn't. The end.
But the middle question is interesting, and I dare say that many people have views such as my own, where morality is not dependent on legality but on morality. And I would also dare to say that most people do not have an issue exposing people who are using websites to commit very immoral actions. So the question becomes one of if the action of cheating is immoral enough to cross this threshold. That question is a really deep moral one and you probably have as much ability to change as most any deeply held moral view.
In short, most everyone agrees the right to privacy has limits based on extremely immoral actions. The difference is only in what counts as immoral enough, which is far less a distinction than 'right to privacy means hacking is wrong'.
I think you have the right approach, but infidelity is not "strongly immoral" in the sense that it entitles you to publicly humiliate people you aren't in any way related to. It's a private matter between two (or actually, more) people. Your spouse has the right to tell you what you did is "strongly immoral", but Joe Public doesn't. I have zero respect for someone from the public who thinks their moral outrage entitles them to shame someone else for who they have sex with.
"Strongly immoral" which belongs in the public sphere is more like human trafficking, murder, corruption in public office, etc.
That's a private matter between the people directly involved. Maybe their partner would also be humiliated by the exposure of their private affairs (I know I would), in which case it makes things even worse.
If you were only telling the person being cheated on, you might have a point. But since you're now telling the entire world that someone was cheated on, or maybe not even cheated on, you've kinda lost that.
Well the point is that the hackers picked a pretty good target. You're never going to convince me and millions of others that exposing cheaters is a bad thing, even to the entire Internet.
Perhaps under your moral framework. But perhaps some other person's framework is different, either requiring a weaker case, seeing this as being a strong enough case, or maybe even having an exception just for a case like this.
Is it legal.
Is it moral.
Should it be legal.
The last is quite complex and I won't get to it here because of the vast differences in moral theories as to what should and should not be legal.
As to the first, is it legal? Hacking isn't. The end.
But the middle question is interesting, and I dare say that many people have views such as my own, where morality is not dependent on legality but on morality. And I would also dare to say that most people do not have an issue exposing people who are using websites to commit very immoral actions. So the question becomes one of if the action of cheating is immoral enough to cross this threshold. That question is a really deep moral one and you probably have as much ability to change as most any deeply held moral view.
In short, most everyone agrees the right to privacy has limits based on extremely immoral actions. The difference is only in what counts as immoral enough, which is far less a distinction than 'right to privacy means hacking is wrong'.