There are all manner of complex reasons why people may sign up to Ashley Madison (and other related sites). Some people are in open relationships while some people may be in all manner of unhappy relationships (those that are abusive, that have mismatches around sex drives). Others may have contemplated doing something, but only thought long enough to sign up to having an account, and never actually contacted anyone. Relationships and cheating are complicated.
This leak has huge potential damage to cause a *lot* of harm to real people. And even more so in countries where extramarital sex isn’t just considered morally wrong but is illegal, or where homosexuality is illegal (some of the data reportedly indicates this). Saudi Arabia is an example of this.
I really hate people forcing their morality/perspective on relationships on to others in this incredibly public and identifiable way. And I think we’ll (very sadly) see more instances of incidences like the suicides reportedly related to the hack.