I can imagine two separate motives for this hack:

(1) The hackers dislike the idea of adultery and wanted to harm the site & punish its users;

(2) The hackers wanted to blackmail the site and its users by threatening to expose them if they did not pay hush money or perform other services (e.g. reveal secrets).

It seems that motive (1) has some acceptance (people are cheering the hackers), but I think (2) seems more plausible. These guys aren't heroes: they found a vulnerable target and went after it.