Look, if someone wants to have an affair they will find a way to do so - whether or not an affair based dating site exists.
And, you know what - everyone involved is probably safer if a site like Ashley Madison is involved - because that way people will use the Internet to get to know each other before they hook up. That's a hell of a lot less likely to introduce an STD into a primary relationship than a bar hook up.
Plus, because both parties know what's going on, the odds of the "other man/woman" trying to break up the marriage or primary relationship is much lower to begin with - because they knew they were getting involved with a married/partnered person in the first place.
Finally, Jezebel and Gawker also constantly complain about how they think it's skeevy that married or partnered people use dating sites like OKCupid, Nerve Personals or Match.com.
Well, guess what - if places like Ashley Madison become mainstream and accepted they won't use Match anymore - because they are just as tired of people being like "ewww, you are a dick/slut because you are here and partnered and you are a creep for contacting me" as you are of being contacted by partnered people.
All in all with places like Ashley Madison everyone fucking wins - so stop giving them shit for simply acknowledging that many, if not most, relationships are not monogamous, even if some married people are willing to pretend to themselves that they are.