It will be interesting to see how this all plays out. The plaintiffs will likely argue that the whole point of the Ashley Madison site is to cultivate secret affairs between married persons - so when the company failed to safeguard the data behind those affairs, they violated the privacy and contractual rights of the customers. On the other hand, there are a host of state statutes which make adultery illegal, so while those statutes are likely outdated and rarely enforced, I'm relatively certain Ashley Madison will claim it had no duty to protect the information concerning what amounts to illegal activity. Good opportunity for creative lawyering.
In another twist, Ashley Madison is sending "take down" notices to sites which are publishing the site's data. Ashley Madison is claiming copyright ownership (or that someone else has copyright ownership) over the data, and in accordance with federal law, is demanding other publishers like tabloid websites remove the data or face copyright infringement claims. Relatedly, because the data does not belong to an individual downloading and viewing it (typically), if you were to download that Ashley Madison database you might also be committing the crime of receiving stolen property. So in a bizarre turn of fate, be weary of the temptation presented by Ashley Madison...it might get you in hot water!