Just a couple things.
1) There's a lot of people on the site who are single and interested in a friends with benefits or no strings attached type of relationship. Married people fit the bill.
2) There's people on there who are looking for a mistress/paramour type relationship, where their well being is taken care of by the married person.
3) There's swingers and people who are in open relationships on the site.
4) Then there's people (like me) who have created accounts just to see what's out there, but haven't followed through on anything.
Just a gut instinct, I'd say the above four points probably make up at least 80% of the website, if not more.
That's a rational appeal to the fairly repugnant people in this thread that condone an illegal and violating action as retribution for something that is neither illegal, nor any of their fucking business.
Beyond that, no one is discussing whether or not cheating is a good thing. It's not, at all. But it's certainly not a black and white issue. Not everyone can end things with a clean break before moving on. It's also about a lot more than just sex.
Passion, and feeling desired, is absolutely necessary to being a complete and happy person. People work on this, and try to make things better, but a relationship is a two way street.
Long term relationships (and I'm talking 10+ years here) are maintained not through inexplicable desire and passion, but through hard fucking work. You keep that relationship alive by not being selfish. By making sure your partner feels desired and loved. By putting someone else's needs above your own. If you fail to do this, that's as much of a failure as the SO that ends up finding that outside of their marriage.
I'm not condoning cheating at that point of failure, but it's an incredibly human response to that situation, given that it's often the result. You don't need to accept the person for what they've done, but you need to understand that they aren't always demons out to just hurt the ones they love by bangin some strange.